Adding a tenant

[Maya V.]

Hi folks, my dream tenant wants to let her boybriend move in. The place has always been rented as a small studio really only designed for one person but will work in a pinch for a couple. Her plan is to move him in this early Fall and then the two of them will look for a larger place sometime within a year.  I don't really have a problem with it but I I will need to renegotiate the rent with her for the extra person and water usage and wondering how to come up with a fair amount for that. 

I also am wondering what I need to do to amend the rental agreement to include him.

SF bay area/$1,300 mo. ~ water and garbage paid by me.

Thanks in advance.