From Bruce's detailed instructions:
7.?Return to the bookmarked page (/yahootransfer).
Click on the big green button at the bottom that says
"Set Up New Transfer."
8. Enter the name of your Yahoo group and the name of your groups.io group in their respective boxes. Do NOT enter any domain information (i.e.: leave off @yahoogroups.com or @yahoogroups.co.uk), just the group names.
Click the big green "Start Group Transfer Process" button at the bottom of the page.?
(You have now completed Step 2 of the instructions)
?
?
--
FAQ on Moving from YahooGroupsGMF Wiki: (unofficial) Help for members (and would-be members) and group managers