Awesome plugin idea. I hope we can work on this one as many people will be using this for WP multisite.
When I try to Create a group on the /wp-admin/options-general.php?page=peters_collaboration_emails.php, I received an error message which says:
Warning: Invalid argument supplied for foreach() in /home/prodin/public_html/wp-content/plugins/peters-collaboration-e-mails/peters_collaboration_emails.php on line 940
I suspect that you should allow your plugin to create your own wp database tables and store that information per blog ID. It should work for normal site and multisite.
Let me know, what you think. Cheers!