You can register fitbit accounts (your own and any other you may have authorisation credentials for) and share fitbit data (sleep, steps and heart) with others based on a "window of time". For example, I may share my fitbit data for the last week of February with my friend. We call these windows of fitbit data "data tribs" (short for data tributaries - like streams of data, get it?).
You can create a data trib by using "Add+" or "Copy". When you use "Add+" you are directed to the fitbit servers for authentication and you must click the "share all" button when at fitbit.com. "Copy" can be used on an existing data trib you created earlier (you can change the dates) but doesn't require authentication with fitbit itself.
When you create a data trib you can call it whatever you want and this makes them good for sharing anonymously. You can give them nicknames so friends know what they are while nobody else can trace them to the original fitbit account details. You can also add some descriptive text when you create them to help other people understand what they are.
No fitbit data is actually stored on our servers. When someone wants to look at a data trib (choosing "view") we retrieve the data directly from the fitbit servers and send it straight to you! We don't use the data at all and we don't even hold onto it!
Our advice is to ensure that when you name or describe your data tribs you do so without revealing any login details at all and if you really don't want anyone to know who the data belongs to then don't say!
All communications between our servers and your browser are encrypted (look for the closed padlock or "https" in the address bar)
All communications between our servers and fitbit servers is encrypted. As we don't store the fitbit data ourselves we can't lose it or allow others to get it.
All your login data and the data we use to authenticate with the fitbit servers is encrypted using the latest encryption services. Even if someone got hold of this data it would make no sense and would not be useful to them.