I accidentally left the Challenge, how can I rejoin?

Unfortunately, as per the Challenge rules - once you leave a team after the Challenge start, you would not be able to re-join.

If you click on the Leave Challenge button, you would see the message below, so if you do click on it accidentally – you would have one more chance to go back.
           

However, if you do proceed and click on Leave Challenge, you would not be able to go back and re-join the Challenge.