A coalition leader, or some sort of vote of the coalition can kick a member like the admin can. The empty country is still owned by the coalition, and if the running game is joined and that country is selected he starts in the coalition.
Also there could be an option to only be able to leave an alliance with the permission of the leader.

If the coalition occupies all the cores of a country, it can make the country position joinable. The player joining starts as a member of the coalition.

Have a lobby in games where players can talk and write messages and the coalition can send invitations to a player to spawn in to a coalition controlled country.