Social Feed Manager allows users to self-sign up for accounts. Those accounts are stored and managed by SFM. Future versions of SFM will support authentication against external systems, e.g., Shibboleth.
By default, a group is created for each user and the user is placed in group. To create additional groups and modify group membership use the Admin interface.
In general, users and groups can be administered from the Admin interface.
The current version of SFM is not very secure. Future versions of SFM will more tightly restrict what actions users can perform and what they can view. In the meantime, it is encouraged to take other measures to secure SFM such as restricting access to the IP range of your institution.