What happens if you have already booked a trip and somebody wants to join you? How can you add them to your existing booking?
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You will need to log in and access your trip page. You can find it under My Trips --> Trips I Joined. Click on the trip name:
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You are now on your trip page. Find the green "Book Now" button in the upper right area of the trip and click on it. 
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The following pop-up will appear:

Just click on "Add Another Participant" and proceed with the booking as usual. Add as many people to the booking as needed.