How to see the members of an Active Directory group in the Mail application
To my knowledge, you cannot do this directly within the Mail.app on a Mac. However, you may be able to accomplish this using Directory Utility (hidden away in /System/Library/CoreServices), depending on how your system is configured. My work Mac is bound to AD and it works for me.

Open Directory Utility. Click on Directory Editor. View "Groups" in node pertaining to whatever your AD server is. You should be able to now search for the AD group you'd like to mail to. Unfold the data for "GroupMembership" and you should see the complete listing of members of this group.

I know this is a bit roundabout, but it does work.