the dedis have a little computer icon next to them. they will always be host and remain host throughout the different games played on them.
people can host their own games using the "create a game" option on top of the server list; as mentioned in the FAQ (
https://halo2.online/help/install ) they'll need to port forward for people to be able to join their games.
if the host quits in the lobby, he can choose who the host can be migrated to. not sure what happens when host just quits in-game.