Board Thread:The Quarters - Q & A/@comment-124.149.107.139-20140630055147/@comment-24150802-20140630055425

When you join a challenge, all of the tasks in that challenge are copied to your own account, and then they are available there for you to do (or not do) at any time that suits you. Your actions for the tasks are unrelated to anyone else's actions. If someone completes their own copy of a challenge to-do, your own copy is still available for you to do.