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Fitbit api allows you to get access to Fitbit user's regardless if you own their trackers or not.
The only thing you need to do is to set up and code oauth 1.0a flow.
Part of this flow needs to involve users who should give your app access to read/modify treir data.
Advanced development skills are required.
More info here: https://wiki.fitbit.com/display/API/Fitbit+API
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