You may have an arrangement where you expect a core set of clients to always be included in a regular group coaching session, but that you still want to reserve a few places for ad-hoc participants. Typically, the regular set would have paid up front and thus should be entitled to a guaranteed place for each session.
You can achieve this very simply by ticking the Auto join option against each player in your coaching group (in the Group tab under My groups). Whenever you create a new group session all those with this option ticked will immediately be set as a confirmed participant in the session. This happens whether the session is set up automatically by the system (due to an auto booking schedule having been set up) or if you set it up manually under the Sessions tab.
So long as you set the number of auto joiners to be less than the maximum number you allow for a session then there will still be space for your other clients to opt into the session in the usual way.
For those clients that have been Auto joined, they'll still be able to opt out if they can't make it - and opt back in if necessary so long as the maximum number has not been reached. Remember that if the maximum number has been reached then a client can still opt in, but they will be put on a first-come-first-serve reserve list and they'll be opted in automatically if enough people opt out.
A client can also set themselves to Auto join in My groups. But this option will only be given to them if they have outstanding pre-paid sessions. As soon as a client has used up their prepaid sessions their Auto join status will be reset.