i think that the practices should be in pre-set teams (and free for all), you should post the practice that day and make the teams when the players post:
[Team practice]
team A:
team B:
when a player post that he can play , you add him to the team A , the second player , to the team B, the third to the A again ... etc etc
so the players just need to watch the post and they know who are his teammates
bye!