Add the ability to name your +1
The ability to name the +1 (or 2 or 3 etc) when RSVP'ng to a meetup. Sure, sharing the meetup via email is great, but sometimes pointless if I'm sitting right next to the person I'm +1'ing. Say for instance, I want to bring my GF and she's in the same meetup group. Would be easier to automatically add her to the RSVP list as my +1 so she gets any notifications and it's added to her calendar.
Good idea in some cases. Though there should be some sort of authorisation. User x allows users y to rsvp on behalf etc.