Hi fcrochester,
Thank you for participating in the Apple Support Communities. We understand you'd like to challenge friends that are using the Fitbit app. Some third-party apps enable data to be synced from the Health app to Fitbit. However, Fitbit does not allow steps from third-party apps to be counted towards Fitbit challenges.
Similarly, Apple Watch supports Activity Sharing with other Apple Watch users, but not with users of other fitness tracking devices. This article explains more about this: Share your Activity with your Apple Watch - Apple Support.
If you'd like to leave feedback for Apple about this, you can do so here: Product Feedback - Apple.
We hope this helps.
Take care.