Distribution lists multiple owners

Hi

We have an Exchange 2010/Outlook 2010 environment. We have some Distribution lists that we want several users to be able to add and remove members.

I have followed the steps in this article: http://blogs.technet.com/b/exchange/archive/2011/05/04/how-to-manage-groups-with-groups-in-exchange-2010.aspx

I got one more user to be able to manage the DL but i have another user that stil gets the error message that she does not have enough rights to make changes in the DL. Another user cannot even she the members of the DL. This is from Outlook.

Is there any permissions i have to check or what am i missing.

Best regards
Matthias