At USC, which has a pretty large greek scene, the frat parties are open to all guys in the beginning of the school year, during rush. I'm assuming you're a guy from your username, but if you're not, girls are always welcome. The rest of the year here for major parties you have to get on the guest list, which you do by knowing someone in the house. Also be prepared to wait in a line for the larger parties. Girls can just walk in, but when things are crowded, there's usually a line of guys waiting to get in, or being turned away.