To add a user to an existing group, select 'Users' on the navigation bar. At the bottom of the page, select the user whom you would like to add to an existing group.
If you want to create a new group, click here for help.
Scroll down further on the page and select the 'Group Membership' tab from the menu. This will list all of the groups that the selected user is a member of. To add them to an additional group, select the 'Add' button.
Next, select the group(s) you wish to grant the selected user access to, and click 'Done'. Once satisfied, select 'Submit' at the bottom of the page.
Click here to see a video tutorial on adding a user to a group.