Give each alliance member one game joining token. He can give it to another member in his alliance.
The second member then joins a game, and uses the token to have the first member join.
The token then goes back to the first member.
Currently, members must be logged into the game at the same time in order to join a game together. This is difficult since alliance members usually live in different time zones. So alliance members cannot join the same game unless the game is so large there is still time to join hours later.
Supremacy1914 will limit which games allow use of game joining tokens.
Supremacy1914 will limit how many game joining tokens can be used in a particular game by a particular alliance.