Can I suggest you send your group an email from your migrated group - by email to your Groups.io email address or by logging into your group space on Groups.io and sending something from there?
Then your group members will see they are still group members and are receiving email from the group.?
I hope most most of your members are receiving mail. My group is set so that Special Notices is the least frequent delivery method. If you have Special Notices only, send your first message via your group site and tick the Special Notices checkbox.?
Frances
Frances