Could one of the devs please chime in here?

There are three things we posited reserve slots in a game:

1) progression between maps—each player in the old map gets a slot reserved in the new one
2) disconnect—the player can reenter the map for a certain amount of time
3) invites—not sure about this one, but Snakespeare suggests that invites reserve slots until all slots are reserved

It would be nice to know canonically how this works, for several reasons. E.g., I frequently get invites to games which I'm unable to join (getting the usual game full message).