Security Group Memberships via Profile
This section defines which security group(s) each user is a member of. When a new user is setup on the system a default security group is assigned to the user called "Read Only". As the name suggests this group will give a new user very limited access to read only Activity Feed items.
Only administrators can grant other users access to a security group.
Security Groups can be setup to give the members read only or read/write access to any single part of the system without granting them access to the entire system. This allows for a very granular security policy.