Click the Manage Online Storage and Permissions link in the Mail panel.
Click Manage Online Storage and Permissions
Click New user to begin the setup for a new user.
Select the E-mail address from the drop-down box that you would like to set up (A). You can also enable access to the 1&1 Online Team Storage at this time if you would like. To allow this new user to have access, click Activated and select a User group and what folder(s) the user should have access to (B) (C).
Click OK to save the settings (D).
You will now be taken back to the overview page where you should see the newly created user and where you can Access 1&1 Online Storage.
For additional information, you may want to reference: