Managing External Attendees
PrimeCal does not send invitations by email, so there's no PrimeCal-native way to invite someone who doesn't have an account — nothing gets emailed to them, and there's no way to add "an email address as an attendee" from inside PrimeCal.
What you'll actually see
If an event syncs in from Google Calendar or Microsoft 365 and the organizer invited people outside their organization by email, those people can appear in the imported attendee list alongside PrimeCal users — see Viewing Attendee Status. That's display-only data carried over from the sync source.
Inviting someone without a PrimeCal account
Send the invitation from wherever it actually works — Google Calendar or Outlook — the same as you would for anyone else. If that calendar is synced to PrimeCal, the event (and whatever attendee data the provider gives PrimeCal) shows up here too.
There is no PrimeCal equivalent of an email-based meeting invite today.