While I can set a maximum number of participants, there is no option to set a minimum threshold for the course to actually take place. This is absolutely essential, as a group course, for instance, only makes sense with at least three participants.
I am currently bypassing the problem by requiring every booking to be confirmed. This allows me to wait until at least three people have signed up. If I were to allow immediate payment upon booking, I would have to refund the money if the course did not take place.
A suggestion for implementing the minimum participant requirement: when someone books a group course, the fee is not charged immediately. It is only charged when the course actually takes place.