To add an existing user to a group do the following:
On the Users tab click on the magnifying glass next to the user you want to manage. This will bring up the manage window.
Next click the Edit icon. This will then open the edit window for this user.
To add the user to a group type the first letter of the group name in the "group" entry box. This will generate a list of groups to select from.
Select the group you want to add this user to then click the OK button to complete the changes.