Hi
Fitbit does not support Apple Watch.
Whilst some third-party apps enable data to be synced from the Health app to Fitbit, Fitbit does not allow steps from third-party apps to be counted towards Fitbit challenges.
For example: Sync Solver - Health to Fitbit
Similarly, Apple Watch supports Activity Sharing with other Apple Watch users, but not with users of other fitness tracking devices:
Share your Activity with your Apple Watch - Apple Support
You may find the following third-party app of interest for challenging your friends:
activity.club
- "Challenge your friends! You can directly challenge friends in activities such as step tracking and runs to bicycle distance challenges and more."
- "AC supports Apple Watch, Fitbit, Jawbone, Garmin, Misfit, Withings, Microsoft Band, Under Armour and more as well as fitness applications such as Apple Health App, Runkeeper, Strava and Google Fit."
- App: https://itunes.apple.com/us/app/activity.club/id903459923?mt=8