Unfortunately, at the moment it is not possible to add or remove Travel buddies for only a part of your trip. This means that both the start date and end date are always the same for everyone who has joined the trip.

If you are only travelling together for a part of the trip, you may decide to create a different trip for each segment, so that you can add Travel buddies for the shared part of your trip.

Alternatively, you can always each create your own separate trips.