Found this out after a bit of Googling, so I'm recording it for posterity here, too:

Suppose you've got a group in the Mac address book for sending emails, and suppose that some of the members have more than one email address. Which one gets used for sending the emails?

To choose (which you can do separately for each group), select Edit->Edit Distribution List from the Address Book menu. Select the group, then choose the email address you want for each member, if the right one isn't already shown in bold. You can select only one per person.

It also turns out that to do this after adding a new group, you have to restart Address Book. Not obvious, but good to know. Thanks to "RT11 Guru" in the Apple forums.

Oh, and apparently none of this works for smart groups. Maybe they are not quite so smart yet.