NICUs have different visiting policies, so find out yours before you or some of your relatives end up feeling very disappointed. Generally, you'll be able to visit whenever you want, and there are often cots or recliners for when you need to rest. If your baby will be in the NICU for a long time, ask your social worker if there's temporary housing available, such as a Ronald McDonald House.
For other visitors, however, there are posted visiting hours. Your parents may be able to come by themselves, but often any visitors have to be accompanied by you (and provide ID once they're there). Typically, NICUs don't allow children under the age of 14 to visit unless they're a sibling, and there are special guidelines even for siblings, like providing immunization records and getting their temperature taken.