I was looking to fill a similar need when I found wiggio.com. Hopefully you will find it as useful as I have.
EDIT: Initially I was looking for a solution to the common "reply all" problem of group members replying to emails, as this group is not using a common mail server/service. We also have members that are resistant to signing up for a new service and Wiggio doesn't require it. You can add users three ways,
Enter the email addresses of anyone you want to add to the group in the Email addresses field.
Select recipients from your email address book(s) by clicking the email contacts link.
Import recipients from a spreadsheet by clicking the spreadsheet link.
You can also create subgroups like
(eg to addresses such as firstname.lastname@example.org or email@example.com)
Should you have the need as we did, you can have multiple admins.
Once our members were exposed to Wiggio, they began to see the value of the additional tools available which you can see below.
From there site:
Wiggio enables group communication and collaboration. Using Wiggio, you can hold group discussions, assign tasks to group members, poll members, share calendars and schedules, upload and manage group files, and communicate using live conference calls, video conferences, chat rooms, texts, emails, and voice mail.