Google Groups setup for administrators - Manual (Page 15)

Create your mailing group with external users
In the Groups page:
1. Click the Create Group button.
2. Create the group as you did in the previous
section with email address and name.
3. Leave the settings as before, but under the
Basic permissions section, now make sure
that the settings, View topics and Post,
include the option, All members of the
group.
This gives your external members of group access
to messages.
5. Train your team 1. Turn on Groups 4. Collaborative inboxes 2. Set up auto-reply 3. Add external users