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) strata , meet 7am Friday at Mother Tongue Cafe (Oakland)