Here's a set of directions for an admin game, which allows the host to change passwords, or set factions back to AI control if someone leaves the game.

https://forums.totalwar.org/vb/showthread.php?t=95420

The host has to do some things to set up the game, then after that all each player has to do is upload their save (there's a filespace here in the ORG, linked in the Throneroom for that purpose). Next guy downloads, puts inhis password, and repeats.