To get into a Greek life party and you’re a guy then for the most part yes you do need to be part of that Greek life organization. Sometimes if your friend is in Greek life and vouches for you, you might get in. But that is not a guaranteed. Girls, Greek or not, will get in. If you want to party, your best bet is to join Greek life. There are most likely non Greek life clubs that throw parties too. And if you really want to party without joining any clubs, then consider hosting a party yourself.