If the contact list is in a shared personal folder, it is handled differently. You can see the contacts and open the list and look at the names, but you can't just use the list in the 'To' field and have it send... does not work. If you copy the contact list to your own personal folder that is identified as an address book, you can do it.
I used a lot of trial and error (with the associated frustration)to find that out.
If you right click on a shared folder in the Contacts Navigator Pane and select "Properties" you will get a dialog with three tabs. If you do the same on one of your own folders, you get a dialog with 8 tabs, one of which is 'Outlook Address Book.' Click that tab and you will see a checkbox labeled 'Show this folder as an e-mail Address Book.' That has to be checked and unfortunately that is not an option in a shared folder.
If you copy the address list from the shared folder into a personal folder with 'Show this folder as an e-mail Address Book' checked you can then reference the list and use it in 'To' or 'cc' or bcc.'
As I have noted before, Outlook is a strange bird sometimes.