To add users to a group make a right mouse button click this icon 
in tray.
Choose "Edit Workgroups" in appearing context menu.
The window of a group's editor will appear.
Add a group in context menu of a right tree. 
By the left-right arrow you can add computers in the group and by the right-left arrow you can remove computers from the group.
To add to the group a computer which name is not shown in the tree of your net make a right click on a right tree and choose "Add User" in appearing context menu.
Enter computer's or user's name in appearing window. You can also add a coment in a round brackets "()"
Press "OK" to save new settings.