To send an address to a different account login to your Pobox account and click 'Remove An Address':
On the following page, Next to the alias you would like to send click 'Send to another user'
A text box will appear. In that text box, enter the address you wish to send that alias to. You can invite either Pobox members or non-Pobox members to use an alias. Click on the 'Send Alias' button to send the address.
Afterwards, you will see a confirmation indicating that an invitation was sent to that address. The address you were sending the alias to will receive an email with additional instructions on how to set up the alias on their account.