I mean if you have an opening and someone wants to join then cool, they just have to wait it out. I dont really see a problem here.
I assume you mean they are in another guild and dont want to leave because there isnt time to join your guild without missing an event. In which case I'd say you either have to wait or the guy joining might miss an event. If you dont want to wait for that person to join recruit someone else that will join now. Their loss, not yours. Life sometimes requires sacrifices.