A user is someone who has completed the registration process for a platform and is currently logged in. They are automatically placed in the platform main group until the p-admin assigns them to another system group.
Users have different rights. Basically every user is able to see those services and content, for who s/he is entitled.
Uses can see those groups which were created by the p-admin, and can choose who they want to authorize reading and writing. But they cannot create own groups.