If you share your #apartment with another person, it's important to have rules in place regarding guests. About.com has some guidelines for what you should discuss before you invite someone to come stay with you.
First of all, make sure you let your roommate know you will be having guests as far in advance as possible. If you have a guest drop in out of the blue, call or text your roommate before they come home so they'll know in advance.
Make it clear to your guests what is okay to borrow or use and what is absolutely off limits. If your guest uses your roommate's stuff without asking, it is your job to fix it.
Set a limit for how long a guest can stay, one that is agreeable to your roommate as well. Also, make sure that the person who comes and visits you leaves things exactly as they found them. Being a good guest includes cleaning up after yourself, and if your guest doesn't hold up their end of the bargain, you need to clean up after them immediately.
Set how often guests can visit. It can be draining for your roomie if they constantly have to be on their best behavior because you are always having guests. Guests should be an exception, not the rule, unless both inhabitants agree on operating as a hotel to out-of-town visitors.