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