How to identify users on Android devices?

Question

I created an app in which the users can do certain things like comment, earn points etc. I need to store these data in an online database (the app is linked to a website), therefore I need to identify the user somehow. I found the following ways:

1. Register/login - I think users don't really like this option because it's not comfortable on a mobile phone
2. AccountManager - I read this has issues
3. OauthToken - I have just downloaded an app that asked me when opening if I give it my permission to use my email address. This permission may scare off many users. Btw, how does it identify the user?
4. Storing IMEI - this is not an option since it identifies the device, not the user, and many tablets do not have an IMEI number

Is there another way to identify the user? I want to avoid using GET_ACCOUNTS permission to get the users email address, the best if I could link a unique id without permissons. I think users would not prefer having their email adress appeared on the screen (as an id) when they first launch the application. (please note that I only want an id, no password is required)

Answer 1

There's a nice tutorial presenting the best practices for achieving what you want:

http://android-developers.blogspot.com.br/2011/03/identifying-app-installations.html

There's also a similar question here in SO:

Is there a unique Android device ID?