Well, this has been brought up before, and my best advice is this:
Unless the GM who's running the event has specifically stated that there's a sign-up limit (not a party limit, that's different) or otherwise assigns special circumstances to quest participation, I would say to just sign up when you know your schedule and then show up at quest time, even if the number of people signed up is already greater than the party limit that the GM has posted.
Now, since most GMs have party size limits, a few things may happen: * You'll show up and half the people who signed up won't show up. This happens fairly frequently. * You'll show up and there will be some sort of raffle or lottery for quest participation. This is no guarantee, but it's a fighting chance at least. * The GM will accept the group's size, even if it may be a bit larger than originally requested.
So don't be turned away simply by the number of people already signed up.