Cal. Civ. Proc. Code § 395.2

If an unincorporated association has filed a statement with the Secretary of State pursuant to statute, designating its principal office in this state, the proper county for the trial of an action against the unincorporated association is the same as it would be if the unincorporated association were a corporation and, for the purpose of determining the proper county, the principal place of business of the unincorporated association shall be deemed to be the principal office in this state listed in the statement.