When you invite people to an event via an MCommunity group, and that MCommunity group is a private group of which you are not a member, each invitee receives an email:
Although the email invitation looks like a typical one, clicking the Yes, No, or Maybe buttons results in an error message:
The Add to Calendar button in the invitation DOES work.
The organizer will not get a "tally" in the event of who has responded/who can attend when using a private MCommunity group. To avoid this, the organizer has to manually add invitees.