I'm sure there could be one, but why would anyone really need to know? You can get a rough idea by going to Player Tables / Recent Movers - I clicked up until the point where the green dots vanish, that was 385 users.
Doesn't help the fact that only 4 of them are my opponents and my other 100 games are sitting idle though!
Well, Player Tables / Recent Movers, you could always challenge or messages someone with a green dot to indicate they're moving now, or post for a game in Meet Opponents, or post an Open Invite.