You have to have 16 players participate in the tournament, so you either need to give
AslanTheKing an extension to join his games or you will need to find a reserve to take his place.
Also, you never actually stated in your rules that players only have 24 hours to join their games. If you just sent out the invitations last night, then you have to wait 24 hours before officially removing him from the tournament. But because you never said you would enforce that as a rule, you should probably give him (and future players that may have this issue) at least another 24 hours to join. Although we only require an organizer to give a player 24 hours to join the game, most organizers will allow a player 48 hours to join their games.
AslanTheKing, each organizer runs their own tournament, so no two tournaments run on the same time-table. If you are a freemium and join several tournaments, you take on the risk of having to join multiple games from multiple tournaments at once. You need to either buy a premium membership or realize that organizers are not forced to make their games around your schedule. If this organizer doesn't want to wait for you to have an open spot, he's not forced to.