1) You add your field to the user table.
2) You modify your existing auth model (not Community Auth's auth model). If you don't have one, make one and declare it.
3) Modify your auth model's get_auth_data and check_login_status methods to return your new field.
4) If you haven't already, extend the _set_user_variables method that is in Auth_Controller by adding it to MY_Controller. Add your new field to controller properties, config values, or whatever you want.