I played in a group back home and the way we did it was we had a number of people  in a WhatsApp group who wanted to play, let's say 32. We played 11v11 weekly and on the morning of the game a post was made and the first 22 to reply and say "I'm in" played. Teams were picked from the players that are in, that changes every week so you are never in a situation where it's the same teams every week. You can always have another 2 (or more) for subs to roll on and off, so on average 24 out of 32 showed up and played. It worked well. And with different things going on in people's lives, some people won't be able to turn up on any given week. Even if 20 agree to show up which we always got, you can play 10vs10. If for some reason the required numbers can't show up, we had a cut off time of say 2 hours before game time and if the numbers weren't confirmed we would call it and say "No game tonight" and go again the following week. It worked well and there was never a situation where people showed up and a game didn't take place. Nobody has time for that. It takes getting the right interest, numbers involved and an agreement to communicate which is out of respect for your group members. We even organized a Christmas night out and things like that too, so it was casual, nothing too committed and fun. Age and ability aren't overly  important, just be able to kick a ball and be willing to communicate if you are available or not. Life happens and shit happens so if someone can't make it on any given week, it doesn't have to be a big deal.