I have three clients that train together and have purchased a 12-session package where the total cost is split in three. If any of them misses with no advance notice she is charged that session. But if she let me know with enough time in advance then she is fine. In order to keep the package intact for all of them and basically make it easier to keep track of I have agreed with them that when this happens, they other two (pay for the session at my partner session rate) or if two miss the one that shows up pays for an individual session. So they all have the same amount of sessions to train together. One of them felt that this was a punishment in a way. It is really not, because she is getting her one on one training and she is paying for what she is getting that day. But I wonder if there is a better way.
Before my clients sign up for a small group training session package they all have to read and sign the policies in place. In the policies I state that if anyone misses a session they will be forfeiting that session and it cannot be made up in a later time. This way I keep things simple and fair for everyone in the group.