owner of DL in exch 5.5

I have exch 5.5/sp3. I have several DL and each have its own owner. But I need to configure the DL in such a way that it can have more than 1 user/mailbox. Everytime I want to add a second user to the DL, it will automatically replaced the first one. And there is only ‘owner’ role in the DL and trying to add another DL to be the owner of the DL will give error.

appreciate the feedbacks.