If you regularly offer group bookings, you need to enable this option in the trip settings to allow the trip to be confirmed by multiple travellers.
(Our product evolves quickly, and this video may not show the latest updates)
How to Enable Group Bookings for All Your Trips
Select the "Group Booking" option
Save your changes
How to Enable Group Booking for a Specific Trip
Go to Trips
Select the trip you want to enable the group booking for
Go to the "Share" step of that trip
Inside the trip's "Settings" section, check the "Group booking" option
Specify the maximum number of travellers who can book your trip, and optionally, the number of travellers who have already booked
Save the trip