Any member of a group can join any of that group's current or future challenges. That is: As long as the challenge is not over and "closed," a person can join the challenge.
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Also, good news: When a person joins a challenge, Count.It will sync all relevant activity data going back to the start of the challenge. Thus, even participants who join a challenge a few days (or more) late will get full credit for any activity that they logged on those prior days.