If it requires a sign-up by a CO (such as FSO) then you must ask another squad that's signed up if you can fly with them. Sometimes you can get there early and ask around, but this may or may not result in your invitation to fly with "squad X"
If it's something like a scenario or a snapshot, you do not need to sign up as a squad. You can sign up individually for scenarios, and snapshots require that you only show up and participate.
What, specifically, were you thinking of doing? There's a whole bunch and they usually have different rules