Personally I don't hound my tenants about long term guests and bringing in people not on the lease as long as those people aren't causing any issues. Tenants are constantly breaking up/getting in new relationships, having family members move in with them, etc. You can keep track of it, but it's a lot of effort for limited gain.
Basically: if someone is causing issues and is not on the lease, I immediately crack down on them, and let the resident on the lease know that they are in violation of their lease and will be also held responsible as well. If someone new is minding their own business, not bothering anyone, and paying rent on time though, I'll put my efforts and energy into other items.
Just my two cents, your mileage may vary.