When composing an email,
1. Click the To button.
2. Click two or more entries from the list of contacts.
3. Click Done.
4. For every contact that has multiple email addresses you'll be prompted with a selection dialog.
5. Now finish your email.

I thinkyou were probably hoping for a real way to create a group inside the email client, but I am pretty sure there isn't one. It's called "Modest" for a reason.

Right now there not really strong and well packaged alternatives, but if you take a look at thread http://talk.maemo.org/showthread.php?t=44287 I think you'll see the energy that will get alternatives in place before too much more time elapses.