Unfortunately, Fitbit doesn't not allow third-party apps or devices to compete in step challenges or share steps with community members (friends). This information is posted at the beginning of the app description.
BUT you can still share your Fitbit dashboard with Fitbit friends and other friends and family using the share option on your dashboard.
More information can be found here - https://help.fitbit.com/articles/en_US/Help_article/2106