Depending on what you want to achieve by having different email addresses for each group, we may can offer a way of doing it; for example, if what you like by this scheme is that you have "clean"/separate inboxes, you can achieve this by adding all the accounts you want to use to your main GIO account as aliases, and creating Forward-then-Delete rules/filters on that main account's inbox to route the messages to the respective inboxes by using the [group_tag] value in the subject line as the "router". So by spending a few minutes, you can do this and the end result will be the same more-or-less as it was on yahoo, inbound group emails still go to the separate inboxes as before; the only difference is that if you reply to one of those routed messages from the aliased inbox, your reply will not show as coming from that account but from the main one.
If you're not familiar with account aliases, read here:
It allows you to send messages to your group from a different email address than your main one. By augmenting that with the inbox filter/rule routing you now get back the ability to also receive group messages to physically-different inboxes, same behavior as was on yahoo.
If this is not the reason for wanting this, let us know.