Make user owner in exchange server 2007 for a shared mailbox

Please help me with below query regarding Exchange Server 2007.

I have created a shared mailbox and granted user Full access and send as access however, user wants to be owner of that shared mailbox. So that user can add and remove other users from mailbox using Outlook.

I have tried marking him as manager in Exchange under mailbox properties -> organisation -> manager.

Please help with this!Image may be NSFW.
Clik here to view.

Raghu Sharma