Depending on your subscription level, you may be allowed sub-users of your account. These can easily be managed in the "Manage Team View," described here.
To access the Manage Team view, select the drop down next to your username in the upper righthand corner and select "Manage Team."
Add Individual New Sub-User
To add a new sub-user, click "+Create Sub-User(s)."
With "Individually" tab selected, Enter Name, Abbreviated Name (Optional), Email Address, and Password.
Select the "Ship From" addresses that should be available to the sub-user.
Set all Permissions and Settings applicable to that sub-user and select "+Create" at the bottom of the screen.
Create Sub-Users in Bulk
With "Bulk" tab selected, enter the number of users and the naming prefix convention you wish to use for the usernames.
Select the "Ship-From" address(es) and apply appropriate settings, then select "+Create" at the bottom of the screen.
Edit Existing Sub-Users
From the "Manage Team" Screen, select the user(s) you which to edit by selecting the checkbox preceding the name in the list.
With at last one user selected, you will have the option of Editing Permissions, Editing the Ship From Address, or Editing Settings.
Editing Ship From Address