A landlord absolutely cannot tell you you can't have overnight guests. To do so would be an invasion of your constitutional right to privacy.
No, a landlord can't impose his approval rights on you and tell you how to run your social life. As my colleague pointed out, it's an infringement of your constituional right of privacy (and also of your freedom to associate).
It's one thing if you have an overnight guest now and then, and it's another if you actually move a new tenant in who's actually residing there and getting mail there and has no other place to live. if your lease specified that you'd be the only tenant and you brought in a roommate, the landlord could add to your rent, but your landlord can't prohibit your guests. This is your rental, and you have exclusive use rights as long as you're complying with your lease.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.