Folks -- I haven't posted this in 8 days so I'm probably overdue to do so again. My apologies for the inevitable dupes in your email inbox.
Appended below please find detailed instructions on how to conduct a transfer using the Easy Group Transfer method.Other resources
Please note that the way you select an existing transfer for further processing has changed. It used to be a pull-down menu, it is now a list of links.
Here is the process. It is not fully automated and will take several days. Do not do anything out of order.
The following instructions are for a full-width desktop/laptop display. This can all be done on mobile too, but it's a lot easier if you don't.
0.2 At the bottom of the page, you'll find a link that says "Easy Group Transfer." Click it.
0.3 Scroll to the bottom of the subsequent page and click on the big blue button labelled "Yahoo Group Instructions."
0.4 You should be at https://groups.io/yahootransfer. Read the instructions carefully, including all the Notes. Bookmark this page, too. You will come back here several times.
1.1 Create your groups.io group and configure it according to your needs. Help with group settings can be found here: https://groups.io/g/GroupManagersForum/wiki/Quick-Start-for-Group-Formation (Note: Each new group must be reviewed to confirm that it meets the Groups.io Terms of Service. This alone could take up to a day.)
1.2 To use the Easy Group Transfer, you must upgrade the group to the Premium Yearly plan. You can downgrade to the Basic plan later (i.e.: a year from now), if you want.
To do so, select Admin>Upgrade from the left-side menu bar and click on the blue "View/Change Plan" button. Select "Premium Yearly" (one-time cost of $220) and make your payment.
(You have now completed Step 1 of the instructions)
2.1 Return to the bookmarked page (https://groups.io/yahootransfer). Click on the big green button at the bottom that says "Set Up New Transfer."
2.2 Enter the name of your Yahoo group and the name of your groups.io group in their respective boxes. The groups.io group is conveniently selected from a pick-list of those groups that you own. Do NOT enter any domain information (i.e.: leave off @yahoogroups.com or @yahoogroups.co.uk), just the group name.
2.3 Click the big green "Start Group Transfer Process" button at the bottom of the page.
(You have now completed Step 2 of the instructions)
3.1 Go to your YAHOO GROUP (not your groups.io group) and log in. Send a membership invitation to "firstname.lastname@example.org". If you've never done this before, instructions can be found at https://help.yahoo.com/kb/SLN2567.html
3.2 Return to the bookmarked page (https://groups.io/yahootransfer) once again. Select the group you are transferring from the Existing Transfers at bottom right (click on its link). A new page will come up, with a status box at the top. If all is well, it should say: "We've received the invite to your group and will accept it soon. There is no need to send another invite." If it doesn't say that, the invitation did not go through. Go back to Step 3.1 above and try again.
(You have now completed Step 3 of the instructions)
4.1 BE PATIENT. Accepting the invitation is often the longest step in the process, and requires human intervention (solving a CAPTCHA).
4.2 BE EVEN MORE PATIENT. Do not send additional invitations in an attempt to hurry things up.
4.3 You will eventually receive an email from groups.io stating that the Yahoo group invitation has been accepted, and providing you with further instructions. Continue waiting until you receive this email.
(You have now completed Step 4 of the instructions)
5.1 Return to your YAHOO GROUP and log in. Find the "email@example.com" member record (sorting by join date is the easiest way) and promote it to a Moderator. Instructions for this can be found at https://help.yahoo.com/kb/SLN2401.html.
5.2 Return to the groups.io page that you bookmarked in #0.4 above one more time. Once again, go to the "Existing Transfers" at bottom right and select the group you're ready to transfer. On the resulting page, click the green "Give Go Ahead to Transfer" button.
(You have now completed Step 5 of the instructions)
6.1 BE PATIENT AGAIN. The ability to actually do the transfer depends on how well Yahoo's servers are operating. We all know what that means, right? (If you want to check status, see https://groups.io/g/GroupManagersForum/message/19948).
6.2 You may notice some activity in your new groups.io group, which probably reflects partial completion of the process. Do not panic. This is what you wanted to happen!
6.3 When the transfer is done, you will receive an email from groups.io summarizing what it was able to transfer. It will also list whatever things it was NOT able to transfer. Until this email is received, you aren't done yet.
Most of us have "been there" with this process before and it all sounds easy in hindsight. Ironically, the same thing that's causing most people to abandon Y!G (server downtime) is exactly the same thing most likely to get in the way of completing the transfer process. The best time to initiate a transfer is when Yahoo is performing well. If you procrastinate until things are crashing and burning, delays, incomplete transfers, and other such frustrations can be expected.
Hope this helps,