Your address book may be using an old version!
In Outlook open a new blank email and start to type the group's name
Select the X to delete the offline version of that contact. This will not change anything on the server or for anyone else, so go wild.
After you clear out the ones you know are bad, click the To... button and select the drop down to navigate.
Typically I'll use the All Distribution Lists list (2), but All Address Lists will work too. If you leave it on the default (1) it won't bother checking the server, so you may end up getting old data.
Once you find a group you want to message, you can right click it to explore its membership.If you are set as a manger of a group you can also change the membership inside of it right from Outlook.
Right now the admin account is the default owner, but it can be changed.