When I click on the "Activity" tab of a Ticket in ticket view, there is a "Member not found" error for guests. But I think moderators/admins could easily be irritated by this not recognizing that it is a guest. Maybe you could change the behavior from a Board Error to just a message: "Ticket Starter sent in the ticket as guest. No activity was logged."
Second option: instead of the solution above you could log activities of guests of course...
Second option: instead of the solution above you could log activities of guests of course...