This one drove me crazy. I was made the manager of a distribution list outside my domain. I did not have the permissions to change the group memberships when I tried to do so through Outlook. There is a KB article for this, http://support.microsoft.com/?kbid=318074.
My Exchange admin sent me the workaround:
Click Start
Click Search
Click Printers, computers or people
Click People in your address book
In the Look In field, down arrow to Active Directory
Type in the name of the distribution list.
Click Find Now
Double click the name of the distribution list.
If you want to remove a member, highlight the member’s name and click Remove
If you want to add, click Add, type in the name, click check name.If the check name does not find the person, click the location tab and choose entire Directory
Once you’ve finished, click OK
Exit Search.