To Add a User to An Existing Access Group: 
  1. Go to Core > Security Content Editor Access.
  2. Make sure Display By Group is selected in the upper-left.
  3. Select Edit to the right of the access group you want to add the user to.
  4. Under Add User, search for and add the individual user to the group.
  5. Select Save & Exit.
Note: The user will need to sign out and sign in again in order to see the change.
User-added image