The host will see players show up under "Players on Network", and should choose "Add player" to add them to the "Players in game" box (or optionally click on the "Autogather" checkbox, and new players will automatically be added). When all players have been added, the host clicks on "Begin Game".

NOTE: If you are gathering an internet game, be sure to share your IP address with all the users that are going to play, and instruct them to "Join by Address" when joining.