Every listing on Hipcamp has a nightly base price with a max group size that a Host sets. For example, a Host may set their base price at $50 per night, for up to 4 Hipcampers.
Some Hosts also allow additional people over the max group size for an additional cost. Additional people can be added at the time of checkout, and modified at any time after booking up until your trip's start date.
If changing the number of additional people after you make your booking, your card will be charged the "additional person cost" - the price per extra person per night, plus a small service fee.
Removing additional people will result in a full refund of the "additional person cost", less service fees.
To change the number of Hipcampers in your booking
You can do this in your Hipcamp account by logging in, hovering over your name, going to Your Trips, clicking on the upcoming trip, and then selecting "edit" next to the group size.