Invite guests to dress up as a favorite character, but don't make it a requirement. It may send a sour note to force people to be something they're not since that would be counter to the show's theme. The "Seattle Post-Intelligencer" suggested making the invitations out of old purple Kool-Aid packets since purple drink is a factor on the show.
Prepare "Glee" themed drinks and snacks. Create purple slushies, and cut cookies or finger sandwiches in the shape of musical notes or instruments. Use decorations to mimic the halls of McKinley High.
After the show and the inevitable discussion ends, do your own "Glee" club. "Glee" has spawned sing-a-long events, karaoke contests and just plain choral singing and dancing. One option is to have each party guest nominate songs the whole group can perform after the show.
Give away goody bags filled with "Glee"-related items. Anything music related will work, or show-related items like vitamins (to represent the medication Mr. Shuster's wife Terri gave the high schoolers), or fake chastity rings (to represent the promise Quinn broke when she slept with Puck).