To change the "User Group" of an existing user:

• Login using an account that has 'Administrator' privileges.
• Click Settings on the top-right, and select Manage Users from the menu.
• In the resulting page, click  edit link next to desired user account.
• In the resulting page, under User Groups section, select the appropriate checkboxes. Then click "Save" button.

To assign a "User Group" to a new user:

Add a new user as explained here. In the "Add User" screen - assign desired group(s) using the checkboxes under the "User Groups" section.