Google-groups – Configuring Google Groups via email

Create a group, from Group Settings -> Advanced - ensure This group is hosted on another mailing list host is checked and provide the email address of the mailing list.

Well, you asked it on 2011 and Google Groups changed recently some features...

But if you want to enable replies from all users on the new Google Groups interface (hey, I use it in Portuguese, so some words here would be synonyms of real options):

1. Enter the group and choose Manage

2. On Details > E-mail Options, change Post Answers to All Members, and press Save

3. On Details > New Members, check the checkbox New Users can Post, and press Save

4. On Permissions > Basic, on Post select All Members, and press Save

5. On Permissions > Post, on Post select All Members (if you have done 4th step, probably this one would be already done too, but once, when I was creating dozen of new groups, options from these steps, 4 and 5, were different, perhaps a momentary bug), and press Save

6. On Info > General, on Post Options, check the option to Allow E-Mail Posting

7. And finally, on Members > All Members, check if someone is not allowed to post (on Posting column). If so, select them and on Actions > Post Permissions select the default option.