Open the Address Book app. Create your new group in there. Close your Mail app, then re-open Mail. Click on the address book icon, click on your new group and select all in the group (cmd+A). And of course, click on "To:" once the contacts in that group are all selected.
I don't know if there's a better/quicker way. But I know this method works cause I just tried it out to make sure.
Along the top of the main window. Might not be there, depending on how you've customized the appearance.
You can customize the toolbar by right-clicking on it and then a big "window" of options will open and you drag the icons you want onto your toolbar. The toolbar is just the top margin of the main window in Mail.