As Dave Sergeant said earlier, this is all about the GDPR EU regulations that come into force on 25 May 2018.? As they are from the EU legislation, they are extremely complex and are of the classic "sledgehammer to crack a nut" variety.
Loads of practical guides will appear on the internet as the date approaches, and I guess we are all going to have to read whichever of those we trust and take a view on whether we risk breaking the law for a low level complexity members' group, or we just say "stuff it" and carry on with life using Groups.io as it helps us run our groups so well.
I know where my vote lies!
Tim Johnson