Click the "Domains" tab on the left hand side bar to open a drop down menu.
Press "List Domains" on the newly opened menu.
This brings you to a List of all domains you have administrative power over.
Next to each Domain 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 Domain" button at the bottom to save your changes.