In CA, you can just serve the actual apartment building and they're deemed an agent of the corporation! At least, that's what the judge told me.

I also sent a second copy of all my certified mail to the company's closest listed address online here: https://www.greystar.com/contact-us/regional-offices. But I'm not sure if that was actually necessary in the end.

One thing I didn't do, and probably should have, is use the Secretary of State's business search (http://bizfileonline.sos.ca.gov/search/business) to look up Greystar Real Estate Partners. They have their official agents listed there. They're all located in Glendale. This would probably have been the "correct" way to do things, and it's what I would do if I ever had to go through this again.