[Wishlist] Invite the anonymous to precise his/her name if relevant
For private events (e.g. birthdays) the organizer may want to accept unregistered participants but approve participants according to their name (rather than accepting fully anonymous people or only registered people).
At long term I would suggest a 3-state event type: anonymous, name required, account required. The second implying a compulsory field "Name and surname" before validating the participation.
At short term though, an easier way to encourage user to leave their name for this usecase could be telling something like "If relevant for this event, you may announce your name so that the organizers approve your participation" here: