Please update all RSVPMaker installations as soon as possible. The wordpress.org plugins team temporarily suspended RSVPMaker from the plugins repository last week because of a security issue, now patched in release 7.8.4. It's now back online in the plugins repository, but that only helps if you install it.
In addition to correcting that one potentially serious SQL injection issue, I was required to add a number of data validation routines that should help prevent future problems.
Meanwhile, please make sure to update. While I am not aware of any active exploits, un-patched installations of RSVPMaker could attract unwanted attention from hackers.
I use RSVPMaker extensively in my own projects and will do my best to make sure nothing like this ever happens again.