Susan, there has been quite a bit of discussion here about this. (Understatement!)? See suggestions about the message archives. And also about inviting members using Groups.io? /g/GroupManagersForum/wiki/Inviting-new-members
But I wanted to say that you could put a message on your YahooGroups saying that your site is moving. Put in the new address.? Before you do that, you might want to consider your setup on Groups.io. /g/GroupManagersForum/wiki/Quick-Start-for-Group-Formation