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If you just want your own user id, you can find it easily by clicking on your profile image in the upper right corner of https://www.fitbit.com/ when you're signed in. Then, look at the URL. For example, https://www.fitbit.com/user/26FWFL would mean my user id is 26FWFL.
In the Fitbit Web API, you can retrieve the user id of the person whose access token you're using by calling the Get Profile endpoint. The user id is also returned as a user_id parameter in the final step of the OAuth 2.0 authorization process.