You may have noticed that you are unable to add mail-enabled Public Folders to groups, as they do not come up listed when you search for them while managing the group’s ‘Members’ tab.
Microsoft does not support the action described above. Active Directories Users and Computers MMC Snap-In will not display Public Folders as valid objects to be added to the group’s ‘Members’ tab.
1) Make the Public folder in question mail-enabled
2) Go to the Microsoft Exchange System Objects container
3) Open the desired Public Folder’s object properties
4) Select the ‘Member Of’ tab and add the desired group(s)
Please note that the function above is only available via the MMC console.