Can a bluegrass band really fill the venerable Warfield Theater’s 2,500-person capacity? It’s a Friday night, and the Yonder Mountain String Band are going to give it a shot. Yonder aren’t just any bluegrass band, though, they’ve become rising “newgrass” stars and have built a steady following throughout the decade. They’ve also got an ace up their sleeve with special guest drummer Jon Fishman of Phish on the bill. The show’s not quite a sellout, but the lower level of the Warfield is totally packed and the balcony’s about two-thirds full. There’s extra anticipation in the air for the appearance of Fishman, who has played with the band before but not here in San Francisco. The Colorado-based quartet’s regular lineup lacks a drummer, but the sharp musicianship from mandolin player Jeff Austin and banjo player Dave Johnston picks up the slack by providing melodies with a percussive feel. Bassist Ben Kaufmann and acoustic guitarist Adam Aijala keep the rhythmic foundation solid. The lengthy first set is well played, but the vibe reaches an entirely higher level when Mr. Fishman joins the band for the last two songs, as a prelude to sitting in for the second set. The energy level soars as Fishman lays down some of his trademark snazzy and jazzy rock drumming on “Robots Jam”, an up-tempo bluesy tune with some minor-key Eastern melodies that lend a Led Zep-ish vibe. The percussive banjo and mandolin playing combine with Fishman’s drumming to send the jam into the stratosphere, while Kaufmann’s bass finds a deeper groove. Aijala seems to really enjoy having a drummer to work with, and the tune is an absolute knockout. This is what the crowd has been waiting for. The set-closing “Steep Grade, Sharp Curves” has a more melodic, major-key vibe, a feel-good mid-tempo rocker where Johnston steals the show — as he will time and again throughout the evening — with ace banjo picking that really seems to gel off of Fishman’s percussion. The band also delivers some harmony vocal work that lifts the song higher. Austin and Aijala contribute their own stellar solos, and the song ends the set in decidedly triumphant fashion. Many drinks have been consumed already, and the bars are jammed at the break. At the beginning of the second set, the band gives a shout out to Bill Graham Presents, the local concert-promoting institution that still manages the Warfield in spirit, if not name. (This, however, is soon to change when the Warfield closes down after the Phil Lesh & Friends shows in May for renovation under new corporate ownership, which has many locals concerned that the Warfield will never be the same.) A smoking rendition of “Angel>Riverside>Angel” opens the second set with all the soloists riffing off one another in some tight interplay where the combustible chemistry between them is displayed. Fishman’s drumming seems to fill a void that drives the band to what must surely be one of their top performances. Tunes like “Cuckoo’s Nest” and “Fine Excuses” take on an extra energy with Fishman’s tight pocket beat and percussive flourishes that provide a platform for the pickers to raise their game. Groove magic is in the air, the Warfield crowd is having a blast, and the boys in the band are milking it for all it’s worth. The only cover song of the evening is Frank Zappa’s “I am the Slime”, where Aijala’s guitar picking highlights another hot jam. But even if one isn’t particularly familiar with the Yonder repertoire, the band’s high level of musicianship looms ever impressive. “Rambler” features some classic bluegrassy riffing that recalls Phish’s “My Sweet One”, which is clearly derived from similar influences. The song features a well received line about taking “just one more Jägermeister shot.” The well-liquored crowd responds with a big cheer as the festive vibe reaches a pinnacle before the band drops into the more reflective “Midwest Gospel Radio.” This in turn segues into “East Nashville Easter”, which builds into another intense jam, propelled once again by Fishman’s electrifying drumming and Johnston’s continually smoking banjo work, before returning back to “Rambler” to end the set. The triumphant atmosphere leads some to ponder why Yonder don’t employ a drummer on a fulltime basis. Two days later, the band show that they can entertain a large crowd without a drummer when they deliver another hot set at their Green Apple Music Festival free show in Golden Gate Park. A cold and windy day forecast is trumped by warm sunshine and Yonder Mountain’s hot licks, as the band caps a memorable weekend by playing two of San Francisco’s finest venues in 48 hours. Kaufmann also fills the bass role in a quartet with the Grateful Dead’s Bob Weir and Mickey Hart, as well as Michael Kang of the String Cheese Incident. There’s little doubt that, after a decade of playing, the men of Yonder Mountain String Band have arrived.