Google Groups – Welcome Email for New Members

Create a user account called forwarding-user@yourdomain.com. Then make forwarding-user@yourdomain.com a member of your group group@yourdomain.com. Then for each of the individuals outsider@somedomain.com who need to email group@yourdomain.com create a filter in forwarding-user@yourdomain.com to foward their mail.

When outsider@somedomain.com emails to forwarding-user@yourdomain.com, it is forwarded to group@yourdomain.com. Messages in group@yourdomain.com go to forwarding-user@yourdomain.com but go no further. For cleanliness, you could create another filter that deleted all the messages from group@yourdomain.com.

Of course, this would be very cumbersome if you have a lot of outsider@somedomain.com users.

Ah, here is how to do it:

In Administration, go to Manage this domain > Google Apps > Setting for Groups for Business > Advanced Setting > Sharing Options

The setting was set to:

Private – No one outside this domain can access groups. Existing external members can only send email to groups.

There is a setting to allow outsiders to see the group.

However, this is global for the domain—there isn't any way (as of this post) to specify this for each group individually. That means that when you enable this setting, outsiders can discover that the group exists even if they can't visit the group. With the setting set to off, no one outside the domain knows that the group exists.