You'll often want to decide when a customer can no longer register for a class. For example it may be that you need to know final class numbers at least 5 hours before the class starts.
Head to Settings>>Registration to get started.
Advanced settings allow you to set different cutoff times for each class.
It may be that you are less worried about letting a customer book onto your popular classes close to the start time vs wanting lots of warning for classes you need to arrange an instructor for i.e. 1 on 1 sessions.
You may also need to let customers register for a class after it starts i.e. Open Gym where there is one long session that runs continuously throughout the day.
Please let us know if you have any questions about this or any of the other registration settings in teamup.