Under Member Notifications, you can add a "Direct Add" notice that goes along with the auto one from Groups.IO.
Mine says something like "We have added you to xxxxxxxxxxxxxxxxxxx as you requested. You should also receive a Welcome email and a copy of our Group Guidelines. Please do NOT click on the Remove Me link below unless you have changed your mind and no longer want to join xxx! If you have questions they can be addressed to xxxxx@xxxxxxxx."
I added that after we had some members who got confused and clicked the Remove Me link, then wrote us and wondered why they weren't getting any messages yet. (The Welcome notice and Group Guidelines are also set up in that same location.)
Barb