You can manage users membership yourself in collections. 

Navigate to the collection you want to manage from omniContent \ Collections, select Manage next to the collection from the drop down option menu.

Select Members section to see a list of members and manage their security.

  1. Click Add Member to add an existing user to collection.
  2. Click X next to the member to delete him from a collection.
  3. To turn on/off specific permission for that member, select Permission and turn it on/off.