Not necessarily MTB, might be gravel or else. 

Ref: https://www.reddit.com/r/BAbike/comments/1c5vx3l/east_bay_cycling_groups/

  * Grizzly Peak Cyclists (GPC), https://www.grizz.org/ The website looks great! mostly road cycling. 
  * Berkeley Bicycle Club (BBC), https://www.berkeleybikeclub.org/. 
  * East Bay Gravel Bikers(EBGB)
  * Waffles: https://www.strava.com/clubs/1000022
  * Mochi: https://www.strava.com/clubs/east-bay-mochi-gang-475243
  * Tunnel Breakfast Club (TBC) [[ https://www.strava.com/clubs/tunnel-breakfast-club | strata ]], meet 7am Friday at Mother Tongue Cafe (Oakland)