Create drop-in group appointments

I'd like to be able to add a group to the calendar before clients are assigned to the group. We need to plan long term and don't know who will attend when we first schedule and plan a group. Currently, a group has to have two members to add it to the calendar.

When creating a group, make an option to add and delete members of the group without having to delete the existing group appointment then go add or delete a client and make a new group appointment.

We run lots of drop-in support groups, which means we don't know who will be there before it happens. We need to be able to create an empty group and schedule it on the calendar with two therapists co-facilitating it. And then after the group happens they can add the clients who showed up, write progress notes, etc.