I get it now....(I don't play paintball so didn't get your frame of reference).
I assume with paintball...that if you want to play you can just grab your stuff, head to a field and join into a game pretty much whenever you know they're not closed. (generalizing)
For airsoft it's a bit different since there aren't any dedicated airsoft fields that play games every day, all day. Hosts that arrange airsoft games have to book in with fields...so the airsoft games are set to a given date, for a given time. So if you were to show up this Friday night at Flag Raiders and say, "I'm here for the airsoft game"...they'd probably look at you strangely.
That said...when there is a game...the "typical" thing to do is:
1. Find and Read the Game thread in the games section...some games have specific rules/conditions/limits/etc...and there is a full expectation that anyone who is there for the game has understood what's put there in the thread.
2. Post in the thread that you're committed to attend. Helps the organizer know how many people (and who) to expect.
3. Be on time (this is not typical since most guys don't seem to own a watch on game day....)
If it's you first game...touch base with the organizer (just say, ""who's running this" when you get there) and let them know you're new to things. And if you're not AV'd...then there's usually an age verifier there too.