California-New tenant would like to add additional tenant

[April Vogt]

I recently rented the front unit of my 1bd/1ba duplex to a new tenant. A few days ago she asked if I would consider allowing her boyfriend to live there as well. I am not entirely opposed to the idea, I just want to make sure I am following the correct procedures. She is about 3 months in to a 6 month lease, which defaults to month-to-month after that. If I do decide to allow the additional tenant, should I re-screen and then potentially re-issue a lease to include both of them? For how long?  I am located in California.