Outlook does not use groups.
What do you mean by group?
What version of Outlook are you using?

Do you mean a distribution list in your Contacts?

This is a bit convoluted, but it works:
--In a new message, click "To..."
--In "Show names from the..." (at the top of the dialog box), choose
"Contacts" (probably the very last entry in the pop-up)
--Select (but don't double-click) the distribution list you want
--Click "Properties" at the bottom of the dialog box
--In the "Members" window, select the recipients you want by shift-clicking
or control-clicking
--at the bottom of the screen, where it says "Add to," click "To" (or "Cc"
or whatever)
--click "Yes," "OK," "OK," to take you back to the message you're composing.

It's easier than it looks.

Ben

Taryn,
When you select a group from your contact list, there should be a '+' sign
next to the name.  Click on that to 'expand' the group (Outlook will give you
warning first).  Then delete the names from the addressing area you don't
want to receive the message

Hope this helps,
---Ron