Many American students are very open and casual about having friends visit and stay in their rooms, including friends of the opposite sex. If that makes you uncomfortable, it is important to talk to your roommate about it and agree on some rules. Your RA can help you and your roommate(s) come to an agreement or compromise if necessary.
If you are unhappy with a situation, talk to your roommate(s) in a calm manner to agree on a compromise. The most important thing to remember is that communication is vital and helps to prevent problems in the future.