However, as our database(s) do not contain non-members
That may be the case for your group but that won't be the case for other groups.
In common with other groups, many of our members are non-pc and have enough problems with using the system - thankfully, the majority now know how to access a database,
If your members are savvy enough to access a database, then 'RSVP'ing to a calendar event should be no more of a challenge.