A lease can say anything, including requiring tenants to dance naked in the parking lot under a blue moon. The real question is if the terms of the lease are enforceable but then you're going before a judge.

I can understand the landlord not wanting non-tenants hanging about but I'm not sure if such a condition is actually enforceable. Does it mean if your girlfriend goes out to the store for a gallon of milk, her guests need to leave with her? Frankly, the comment about the bikini should be enough to have her looking elsewhere.

She can't have guests when she (your girlfriend) isn't home? or when the landlord isn't home?

As a single woman, the bikini comment would have caused me to cross that apartment off my list completely, no matter how nice it was. Totally inappropriate.

Well, if both of your names are on the lease, then you aren't a guest. But I'm with mike and camlan, after a remark like that I'd be worried about the creepy landlord coming around when your girlfriend is alone. And remember, he'll have access to the apartment.

To answer part of your question, that rule isn't common as far as I know.

But there often are restrictions about the number of people who can live in the apartment. The limit is usually two people per bedroom. And their names may have to be on the lease and their cars may need to be registered with the leasing agent.