The problem has been around "from day one", because ev1 has to sign up to play. Some end up in a CC immediately and lose interest in the whole thing the next day. That's why many CCs demand a higher tier - they may lose interest at some time but probably not so fast!
If you're willing to take them, stay easy and compliment those out smoothly who do not play. Perhaps explain the rules of engagement when they join, or use the message that you can send with the dismissal.
You might want to purge those who haven't played for the longest time regularily, when the CC becomes full.