Can my landlord dictate rules on visitors? (San Francisco)?

In the lease agreement, there should some reference to "subletting". However, an overnight guest is not intended as a roommate and therefore does not fall under this category, unless of course in the lease agreement it was actually worded that there cannot be "overnight guests'! Usually on a lease there's wording something to the effect of "if there's a guest that stays for more than a week or so, then perhaps the land lord can enforce a rent increase or subletting policy or even eviction!

If it's in the rental agreement, it may be enforceable because signing a lease agreement is signing a legally binding contract, and violators may be subject to eviction. However, according to 6.15D of the rent rules and regulations, two people can live in a bedroom legally, according to housing code, but only if those people are registered as family or domestic partners.

If it's not in the lease agreement, I don't think it's enforceable unless the lease agreement is amended with the new rule and resigned by both parties.

Of someone is living there, and the landlord does not know. Like a friend, a girlfriend.

The landlord is entitled to charge them rent.

As they are occupying the premises and using it as their residence.

Yes it is.

An overnight guest isn't a visitor.

He is renting rooms to one person, not however many you want to cram into a room. He doesn't care if you're having sex, he just doesn't want to rent to four people and have them turn it into 8.

If you signed on it, he has a good chance to enforce it. Your landlord is a DICK