Good luck on this but everyone I know just uses Facebook to coordinate events. Plus Facebook Mobile lets you keep updated even on the go.
I just created a test on anyvite and it was super easy and common-sense. Definitely preferrable to facebook in my opinion. Two comments though:
1. RSVP dropdown is smallish/hard to notice. 2. Background images stretch in an ugly way, at least on my work IE 1024 x 768.
Otherwise, a great site!