Can I checkout a group appointment individually?

Absolutely, to do this:

1. Click on the Calendar icon on the side menu
2. Find any appointment in the group and click on it
3. On the relevant appointment, click on the Actions dropdown and select View appointment
4. Click Pay now
5. Click the relevant payment method
6. Select Complete Sale 

You can either repeat this for the rest of the appointments, or you can checkout the group altogether and only the remaining unpaid appointments (without sales) in the group will show.