If you want to send a message to a non-member of a Collaborative Inbox, you can do so by adding the address you wish to send the message to in the Cc area of the message.
To send a message via a collaborative inbox:
Open the group you wish to send a message from
Click on New Conversation on the left-hand side of the screen
Click on the From drop-down arrow and choose to send the message from the group rather than your own email address
Click on the Cc link on the right-hand side of the message
On the Cc line, type the email address you want to send the message to
Enter the subject and message
Click on Post Message