The California Code of Civil Procedure sets forth the general rules governing the service of summons.  Cal. Code Civ. Proc. §§ 415.10 - 415.95.  Chapter 17 of the General Corporation Law adds to these provisions.  In some cases, the form of the business may be unknown.  As it turns out there is a statute for that.  Section 415.95(a) provides:

A summons may be served on a business organization, form unknown, by leaving a copy of the summons and complaint during usual office hours with the person who is apparently in charge of the office of that business organization, and by thereafter mailing a copy of the summons and complaint by first-class mail, postage prepaid, to the person to be served at the place where a copy of the summons and complaint was left.  Service of a summons in this manner is deemed complete on the 10th day after the mailing.

Before relying on this statute, however, it would be prudent to check the Secretary of State's online business search tool because Section 415.95(b) provides that service of a summons pursuant to Section 415.95 is not valid for a corporation with a registered agent for service of process listed with the Secretary of State.