Good question! There is not yet a tool in Hipcamp that lets you specify that there will be more or fewer people joining you on one night of your trip vs another night.
Our recommendation is to select the group size that reflects the maximum number of people who will be in the group at one time and then make a plan with the Host to get the correct refund for the days that you had fewer people in your group. The Host has the ability to refund this amount from their calendar after your trip ends.
Our Support Team is here to help with any refunds that need to be processed in this way if your Host needs assistance as well. You can file a support ticket and we can help with getting it squared away.