When you check a contact into a group, the group appointments for that group also feature into the Appointments section of that contact, even if s/he did not actually participate. If you cancel that group appointment in that contact's Appointments section, it will be marked as cancelled for all of the other members of the group, even those who did participate, so we do not recommend doing that, because it will also disappear from your Certification Report, which will make you lose actual coaching time.
If you generate and look in the Certification Report, you will see that the group appointments are specified as such and only appear for one of the group members (first person alphabetically in the group). So, if you had a 1 hour group appointment with 5 or 10 members of the group, the appointment will be featured only once in the Certification Report for the first person alphabetically in the group and only 1 hour will be counted, not 5 or 10 hours. And this is how the system keeps an accurate track of your coaching time.