California Code of Civil Procedure 872.310 is the California partition statute that specifies how a summons for a partition action shall be served upon both named and unnamed parties. The statute provides that:
(a) The form, content, and manner of service of summons shall be as in civil actions generally.
(b) Service on persons named as parties pursuant to Sections 872.530(b) and 872.550, and on other persons named as unknown defendants, shall be by publication pursuant to Section 415.50 and the provisions of this article.California Code of Civil Procedure 872.310
Usually, this means that the co-owners are personally served. However, in the event that co-owners cannot be located, they can also be served by publication.