Serving an Angent of Service - Business

I need to serve an Agent for Service of Process, however he's out of the country. He is part of a rather large law firm (his name is on the law firm) and many other lawyers have worked on cases involving the same defendant. Do I need to wait until he comes back? Can I serve someone in the law office? What's the best way to proceed?

This is in California.

Serving an Angent of Service - Business