If you need to send a message to just some users in a Global Directory List, you can search for the users belonging to the list then deselect the people you do not wish to include.
Start by opening the Address Book from the tool bar of a new e-mail message.
Open the "Advanced Find" menu from the address book and enter the criteria to search for the users in the group you are looking for. If you need more information on how to search see the following article.
E-mail - How to find users that belong to a Global Distribution List (Site / Staff)
Once you find the results of the list you want to message you can Shift+Click to select the entire list or Cntrl+Click to unselect specific users. Once you have made all of your selections click the "To" field at the bottom.
This will fill in all of the names selected above.
Click Okay and all the users will be in the send to field for the message.