To create a group/query of attendees to an RSVP event:
  1. Go to Data Management > Query.
  2. Create a new query with a name and description.
  3. Select to add an Interaction Clause.
  4. Select Default Types > Event RSVP.
  5. Find a user with this interaction and enter the subject of the interaction in the blank provided.
  6. Once you run the query it should populate all users with this interaction.
  7. To add the results to a group > Use query and choose to add results to a group.
  8. Once completed you can select the query to rebuild on its own to add users daily or weekly.
If you are not getting all the results you expect, make sure the name of the event has not changed.  If it has include an interaction clause with that subject as well and use an "or" statement.