Click the "Users" tab on the left hand side bar to open a drop down menu.
Press "List Users" on the newly opened menu.
This brings you to a List of all users you have administrative power over.
Next to each User name is a green box that says "Edit" in it, click this to open the edit menu for the desired domain.
This opens a screen where you can change passwords, grant and restrict services, or change the group assignment.
Once the settings have been altered, press the large blue "Edit User" button at the bottom to save your changes.