I was troubleshooting an issue with IG Group Owners not being notified when someone joins or leaves a group. I was able to fix it, but in the process I needed to wipe all the old notification settings that were in place (otherwise, people would get spammed to heck and back from joining / leaving groups).
PLEASE TAKE A MOMENT TO GO TO YOUR GROUPS, AND RESET YOUR NOTIFICATIONS.
To choose what you get notified by, go to the gear in the upper Right corner of the group, and choose Notifications from the menu:
By default, you will not be notified of anything unless you choose to. Group Owners & Moderators will have more options to be notified of, than just regular members.