For a demo it sufficient to use demo mode and don't bother about security.
If you need to show Mendix security capabilities (like access rules, 'User with roles 'SmallDecisions' can only see items with a price < 100 Euro). Put it in product mode, fill in users, roles and access rules. Normally you don't care about sessions. Mendix handles that.
Edit: A complete module with 'register', 'forgot password' etc is developed by several people but not yet available in the appstore.