On an intermittently rainy Sunday, half the participating members of this week’s Capital Area Bluegrass and Old-Time Music Association jam are playing inside Arlington County’s Lyon Park Community Center, while the other half of the 15-odd musicians are outside jamming in the park. In the middle of the center’s 90-year-old reception hall, Bobby Smith sings and plays guitar alongside stand-up bass player Nancy Lisi and guitarist Robert Swain.

Smith, who looks as though he walked straight out of central casting in a long beard and overalls, drove up to Washington from Kentucky that weekend for the rally at the World War II Memorial. “I took a photo with Sarah Palin this morning,” he said. Smith found CABOMA through an Internet search of bluegrass jams, Googled the location and showed up with his guitar.

Welcoming drop-ins is par for the course at the CABOMA jam, which embraces experienced and novice musicians. Mary Romagnoli, who plays mandolin and fiddle, has participated for several years. “It’s a safe environment to play, and on the other hand, there are some really good musicians that show up. There’s a kind of a game you play — you pick a circle [of musicians], and if the circle’s too good for you, you stay on the edge. . . . You can do anything here. A project, a song you want to roll out — and it’s just fun.”

CABOMA got its start about 30 years ago, when a group of local musicians started meeting in leafy Lyon Park to play bluegrass and old-time music (acoustic American folk music, played primarily with fiddle and banjo) together. That informal beginning evolved over time into an organization that now has 150 dues-paying members. As many as 45 participants show up at the twice-monthly jams in good weather (as few as 10 in poor), with three or four circles of players at different skill levels spread out across Lyon Park’s grassy open space.

Musicians still finding their footing in playing bluegrass are encouraged to come for the first hour and play in the “slow jam” led by an experienced member; the group’s Web site, caboma.org, includes a songbook for musicians new to the genre, which includes many of the songs that might be played at a typical jam: “Red Haired Boy,” “Whiskey Before Breakfast” and “Roll in My Sweet Baby’s Arms.”

Passers-by are welcome to listen in, and children from Lyon Park’s playground sometimes drift over to watch and occasionally dance along to a tune or two. But the jams are meant less as performance than practice, a chance for musicians to hone their playing alongside fellow musicians.

Within the area’s bluegrass community, jams are an informal social complement to more formal gigs and performances — Lisi and Swain, for example, are two of the four members of the King Street Bluegrass band who played three gigs the same weekend they showed up at CABOMA to let off steam and jam informally. And while the D.C. Bluegrass Union lists multiple jams in the area, CABOMA is the most well established — and, as Romagnoli notes, it’s “centrally located, easier to get to, and there’s a Facebook group that promotes it.”

Social media have brought new followers to the CABOMA jam in recent years, although its core group has remained fairly constant. “It’s been good for keeping us going,” said CABOMA’s president, Bruce Day, who started a Facebook page for the association three years ago. “We’ve gotten new blood.” Day said social media appeal to longtime members as well, with some older musicians saying they decided to sign up for Facebook to join the group online.

The association’s latest challenge is finding a temporary home when the Lyon Park Community Center will undergo renovations this winter. Local public radio station WAMU (88.5 FM) has offered to make its American University studios available to the group, but it’s unclear whether many of CABOMA’s members will be willing to drive to downtown D.C. and play in a space that’s exclusively indoors. With several circles of musicians playing simultaneously, this could prove acoustically challenging, “especially with the banjos,” Day notes.

The jams give rise to musical friendships that extend beyond CABOMA’s sessions. Mandolin player Alex Orr recently formed a band with several bluegrass musicians he befriended through CABOMA. That group now performs regularly at the Bethesda farmers market, among other venues.

Smith, the Kentuckian who was just passing through, ended up staying an extra day to perform at King Street Blues in Old Town on Monday night with Lisi and Swain, neither of whom he’d met before the jam, before heading back home. The spontaneous connections between the musicians attest to the joy of playing bluegrass and old-time music together.

“People love the music, they really do,” Orr said. “People around here have been doing this as a hobby their whole lives. One of the things I love about this is that it shows that people should play more music.”