Re: Solving the overload issue (Selecting specific messages into two batches - Individual email vs. Full featured digest delivery) #suggestion
My responses below should not be taken as a disagreement with your suggestion, rather they are advice on what you can do with the system today. There are multiple approaches, but discussion of existing features is really off topic for beta, so I invite you to ask in GMF if you want more detail.
Unfortunately again and again members leave because they feel floodedThis has been an issue in one of my groups, GMF, as well. My general recommendation is that members feeling flooded should check out the Advanced Preferences section of their Subscription page on the group.
So are group is loosing many members, they feel flooded with uselessI've added advice about the use of Advanced Preferences to GMF's Goodbye member notice, inviting them to rejoin if they wish, or simply monitor the group's messages on the web without joining (GMF has public messages).
1. The messages which contain "important medical information" whichI agree with the moderator that using Special Notices seems like a good fit for this use case.
Another would be the use of a subgroup, with the primary group reserved for important moderator posts only (Announcement-Only Group). But that is a Premium feature so may not work for you.
Now you could say those members should choose themselves, if they wantYou can use the Default Sub Settings tab in your group's Settings page to make the default subscription Email Delivery be digest or daily summary. That won't change any existing members, but new members would start out the setting you choose. You could also choose for them a Message Selection option other than "All Messages".