Pobox Plus and Mailstore accounts are able to create delivery groups, which allow different aliases to deliver mail to different addresses.
For example: If you have two aliases, your first alias could deliver to one email address, and your second alias could deliver to a different email address.
This is different from a Pobox Basic account, where a group of aliases will always go to the same delivery locations.
You can edit your delivery groups here.
A delivery group is part of your account, and uses all of its settings -- including password and spam filters. As such, it is typically not appropriate for someone who wants to use it as their primary email address. If you want someone to have a separate account, with their own password, you can add a new user to your billing group.