I've been pushing use the of app to my group members and it has had a positive response. I've had comments saying that as they get the notifications they need and read messages via the app, how do they stop the email delivery? From their point of view, the email is redundant now. I did some testing and realised that notifications are only sent in cases where Groups.io emails you the text of a message at some point so that means if you have subscription settings of Individual Messages, either of the Digest formats or Special Notices. If you have a Daily Summary, the text of messages is not included so no notifications are sent for messages that would be listed in the summary. I have the 'No email' option disabled in my group so I expect you don't get them then either. As far as I can see, the simplest thing a member can do to still receive notifications via the app but reduce email traffic is to select one of the digest formats.
I've been thinking about this as well and want to fix it. But I haven't figured out how to tweak the subscription settings to allow for a setting where you get notifications for individual messages but no emails. Do you have any thoughts on that?
(who has been busying working on the app this week for other reasons)