Responding to Invitations
Not supported today
PrimeCal does not have an RSVP feature. There are no Accept / Decline / Maybe buttons on an event, no pending-invitation state, and no way to change your own response status from inside PrimeCal — for an event you created yourself or one synced in from elsewhere.
If you were invited to a meeting
Respond in whichever calendar app the invitation actually arrived in — Google Calendar or Outlook. That's where the RSVP genuinely exists and where the organizer sees it. If that account is synced to PrimeCal, the event and its current attendee status (including your own response) show up here too, read-only — see Viewing Attendee Status.
Responding inside PrimeCal does nothing, because there's nowhere for that response to go.