Sticking my oar in - the RSVP is fine if you are asking if folk can attend on a specific date, but since this still seems flexible I wonder if a table under www.doodle.ch might help to give a clearer overview of who can come when. It runs in English as well as German. I have never made one myself but it might give you the chance to give more concrete options.

You can have this post deleted if it's out of line.
Hope you enjoy yourselves anyway.