The four members of Bombadil bound like bunnies across the 2-by-6 wooden planks of the Grove Stage at the Shakori Hills GrassRoots Festival. When Bombadil gets loud, its members hop frantically from instrument to instrument, blasting saxophone, trumpet, glockenspiel, shakers and organ into a solid rock foundation of guitars, drums, bass and piano. And when Bombadil becomes quiet, acoustic guitars drift, settled drums shuffle and gentle harmonies emphasize a hushed sentimentality. The contrasts are sharp, the dynamics high.

The young Durham quartet is playing the biannual roots-music gathering in the woods of Chatham County for the fourth time in the last three years, and the 9:15 p.m. Saturday slot is the best billing it has ever had here. Watching Bombadil tonight, it’s clear that the band of 24- and 23-year-olds has earned its place through three years of touring in any place that would have themschool cafeterias, gyms, dive bars, restaurants. Tonight, a bass strap breaks completely and has to be held together with scavenged duct tape. The black plastic top of a piano’s sustain pedal rips from its hinges, so the three members who alternately play the hulking keyboard must step precariously on a fragile spring with their boots. And the rain dousing the crowd sweeps across the members’ faces in thick sheets of spring mist, slickening guitar strings and causing the band’s sweaty brows to drip at double-speed.

Through it all, they smile, sing, shout, improvise, exclaim and make one thing clear: When Bombadil is at its best, which is almost always on a stage, it can make any crowdbe it a soaked gaggle of high schoolers and parents at an outdoor music festival or a smoking, drinking clutch in the burrito bar of a mountainous college townforget most everything else for about an hour.

As much is scribed across the face of Lindsey Moody, a 15-year-old girl from Siler City, and her 16-year-old friend, Jake Sessoms. Long before the first song of Bombadil’s set begins, Moody and Sessoms press against the stage, smiling and staring up at Bombadil’s members. Stuart Robinson stands behind a piano. Daniel Michalak straps on an acoustic guitar and heads to the middle of the stage. James Phillips sits behind drums. Bryan Rahija grabs an electric guitar.

As a stage manager introduces Bombadil to the crowd, Moody tries to predict the first song. Hearing an early piano chord, Sessoms calls out “Smile When You Kiss,” from the band’s first album, A Buzz, A Buzz, which isn’t even for sale yet. They grin. She sings every word.

Soon after the storm intensifies, Sessoms reaches into his pocket and pulls out his cell phone, cupping it to protect it from the rain. The pair’s ride back to Siler City doesn’t want to contend with this weather, it seems, so the rest of their party waits in the car, ready to go. Moody is defiant: “No! Just no. Tell her you’re at Bombadil,” she says, employing the sort of well-duh intonation that makes it clear, for her, this up-and-coming, hard-touring, vibrant Durham band is as important as the biggest radio name.

Sessoms timidly offers this explanation into the receiver and hangs up: “Man, I’m going to get in so much trouble, Lindsey,” he yells, more concerned with the punishment than the buckets of rain pouring across his wire-frame glasses and onto his Allman Brothers Band T-shirt.

“Who cares? You’re at Bombadil,” she says, doffing her head up to the band and back to him, making it all sound so simple. “Just tell her that.”

He’s exasperated but excited. Grabbing a wet festival program from the stage and plopping it atop his wet hair, he smiles. She’s right. In unison, they laugh and launch into the first verse of “Johnny.”

Stuart Robinson sings the piano-led number, the only song recorded both for the band’s 2006 debut EP and for A Buzz, A Buzz, which will be released April 29 on Concord-based imprint Ramseur Records. Robinson wrote “Johnny” three years ago about his college roommate, who “cut himself to stay sane” after a breakup. But, in Bombadil’s hands, the song feels as light as the subject is dark: Three-part harmonies sweep in during the chorus, and triumphant horn flares fill the space of an instrumental midsection. The drum patterns are whimsical, and the piano that pushes the melody is springy and direct. They’re feeding “Johnny” their own youthful hope.

“A lot of people ask us about that song, but we never really tried to make it that way,” said Robinson, standing beside the stage two hours before the Shakori Hills set, as local bluegrass collective Big Fat Gap arranged its microphones onstage. “I guess that just says sort of how messed up we really are. The four of us, I mean.”

Bombadilfull of personality, fattening its lean roots rock with South American accents, vaudeville flourishes and melodic panachefeeds on its idiosyncrasies and contradictions. The band is named for Tom Bombadil, a secondary character in J.R.R. Tolkien’s fantasy universe, but no one in the band has read any of those books. Phillips plans to read them this year, but, in April, his New Year’s resolution remains just that. A schoolmate at Duke suggested the name and even penned “Julian of Norwich,” a Bombadil song that lifts the name of the 14th-century English mystic for its hook. The band named one song for La Paz, the capital of Bolivia, where Rahija and Michalak started writing songs together in 2004. There’s a “Kuala Lumpur” and a “Rosetta Stone,” references to Mark Twain’s river men and pirates, cavaliers and caterpillars.

Phillips has degrees in history and Southern studies from UNC-Chapel Hill. Robinson graduated with an economics degree from Duke in 2006. Rahija studied public policy at Duke, and Michalak earned dual degrees in Spanish and religion. Michalak’s brother, John, was the band’s original drummer, but he quit late last year to concentrate on getting admitted to medical school. Phillips found Bombadil through an advertisement that requested a drummer who must love burritos.

“I guess John got tired of little burrito bars,” Michalak says of his brother, smiling in the second row of seats in the band’s new van. Bombadil’s driving to a show in one such burrito bar in Boone two days before the rainy Shakori Hills appearance. In the back, Robinson lifts a February copy of Forbes from his computer bag. During the red-eyed drive back to Durham, he sorts through a series of on-screen spreadsheets for three hours. Rahija explains the intricacies of Sicilian playing cards (no queens, only naves), and Michalak shares magic tricks and a story about buying a deck of Magic cards at a comic store in Delaware.

Indeed, offstage and in street clotheslocal band and Duke University T-shirts, blue jeans, tennis shoes, sandals, a college hoodiethe members of Bombadil look more like fresh-faced undergraduates than young rock musicians sharing a label and occasional stage time with one of the country’s more masculine and best-dressed bands, The Avett Brothers. Robinson has a smattering of a pencil-thin mustache, and Rahija’s face is lined with the beginnings of a new blond beard. The most boyish and affable, Michalak wears his hair in high, seemingly windswept spikes. With thick red hair and a matching beard, Phillips looks like he’s awaiting his first professorship.

But when Bombadil hits the stage, they instantly seem twice as big as the members’ friendly, unassuming offstage natures suggest, twice as experienced as that facial hair lets on, and twice as strong as the weakest parts of A Buzz, A Buzz indicate. It’s an album that succeeds for its quiet-loud contrasts and charming wordplay but sometimes mismanages those attributes as overindulgences. Live, it’s one long victory parade.

Part of it is a fashion, certainly: Bombadil travels with uniforms they recombine and embellish before every set. Rahija dons riding boots and a simple blue suit, and Robinson stands tall as a member of Southern High School’s marching band in full game-time regalia. Phillips wears parts of a five-piece suit, and Michalak wears a scarf with a matador jacket. They all wear fancy hats. But as Michalak runs between microphones wailing on harmonica or Robinson plays trumpet with his right hand and piano with his left, the band’s answer reveals itself as the willingness to do whatever it takes to make the songs stick and to entertain. The band agreed to be energetic on stage near the start, but finding the line between energy and annoyance has been one long tripwire of trial and error.

“I usually don’t like live music,” says Michalak. “It’s boring, and people don’t move. Maybe I just don’t like loud things. So I try to do something that people like, andif they don’t like the musicmaybe they can still have fun watching us move around.”

Robinson remembers one listener telling them to calm down, that the band was more focused on its own energy than using it to power the songs. They listened: “I don’t remember really having an emotional response to that statement. But it was at a point where we had all agreed to be really energetic onstage but hadn’t really learned how to hone it or develop it.”

“I think we’re just learning more and more. Our personalities are so tied to our music, it’s nice for people to understand one part so they can understand the other, to understand where we come from,” says Rahija. “They feed off of each other.”

Or, as Lindsey Moody told Jake Sessoms one last time Saturday night during the middle of a Chatham County thunderstorm, “Tell them: Do not leave.”

Bombadil plays a CD release party at Cat’s Cradle Friday, April 25, at 9 p.m. with Nathan Oliver and The Love Language. Tickets are $8-$10.