If user1 creates a new event shared calendar of user2 and adds attendees to the event, no invitation will be sent.
Steps to reproduce

1. User2 shares his calendar with user1, giving user1 full access rights
2. User1 creates new event in shared calendar of user2 and adds an attendee to the event (with an external email address / invites user that is not part of the SoGO instance)
   Expected result: attendee receives invitation via email
   Actual result: attendee does not receive an invitiation via email
   If user2 delegates his mailbox to user1, the result is the same (no invitation mail will be sent).