I'm sorry I didn't find this information earlier. I'm posting it now as an fyi, and a possible reminder to post updated information when our next election day comes.
Voters Experiencing Homelessness Fact Sheet is a fact sheet prepared by the California Secretary of State's office. Basically, it explains that any eligible voter can register to vote as long as they have a mailing address. The mailing address can be a PO box, a friend's house, a shelter that agrees to accept mail, etc. The voting address, needed to assign you the appropriate voting options on your ballot, can be cross streets or a park, as an example. See the fact sheet for more details.