Make the error message and validation more explicit for invitations
https://sentry.ewi.tudelft.nl/organizations/eip/issues/1245/?project=6&query=is%3Aunresolved Error is triggered if invite is already sent.
https://sentry.ewi.tudelft.nl/organizations/eip/issues/1245/?project=6&query=is%3Aunresolved Error is triggered if invite is already sent.