Build backward from the moment you have to be somewhere else. Everything else follows from it.
A group timeline is not a list of appointment times. It is one deadline, a running order, and a buffer. Most group days that go wrong go wrong because someone wrote down start times and assumed the end would take care of itself.
Write down the time the group has to be somewhere else, then work backward. Give the location that time rather than a preferred start. They know how long their own services take and how many providers they can run at once. You know when you have to leave. Those two pieces build the schedule between them.
The number that matters is when the final person in the running order is done. Averages hide the problem. Ask specifically what time the last person walks out, with your real head count, and build the rest of the day after that number.
Somebody in the group always has another job that day — the person who is being photographed, the one collecting a grandparent, the one who has to be back at the office. Put them early in the running order on purpose, and tell the location which person that is.
Someone arrives late. Something runs long. Leave slack between the last finish and the deadline instead of stacking them, and name one person in your group as the contact so the location is not calling four phones when something moves.
Tell us the occasion, how many people and when you need to be done. We will find the locations that can hold the group and get the terms in writing.
Book the GroupA head count and a date is plenty to start — we will follow up on timeline, deposit and what the location needs to know.