This app was mentioned in 3 comments, with an average of 3.67 upvotes
Fitbit doesn't allow it.
If you can get them to download the Stridekick app, you can do challenges that way. That is what I do with my Fitbit friends!
Edit: Link: https://play.google.com/store/apps/details?id=com.stridekick
Stridekick haven't tried it myself but my brother in law has a garmin and we were thinking of it the other week. Totally forgot about it though.
Yes, it does. Stridekick
Link me: Stridekick