That said, I've been thinking about ways to increase engagement. I*wary* - or perhaps you _are_ simply weary. ;-)
I agree with you about starting "sponsored" groups. Better to find ways to recruit subject-matter enthusiasts to create "organic" groups.
In terms of categorizing groups, that opens a big can of worms.Yup. Often a group fits more than one category, or in a select subcategory that isn't in the taxonomy (yet).
I like the idea of tagging better, especially if there's a facile way to suggest existing tags and provide tag descriptions while the group manager is adding tags. On the other side of the coin (a user looking for a group) something like a tag cloud could provide a way of exploring related tags (tags that are used together in more groups are more closely related than tags that occur together in fewer groups).
Also, I've been banging my head with trying to think of ways to makeYour existing sign-up mechanism is so easy to use that I'm not sure you need to do anything beyond letting the Yahoo or Google Group managers post the Groups.io URL or +subscribe address themselves.
Easier still may be for the Yahoo or Google Group manager to make use of your one-time Add fee - but that would depend on the relationship between each group manager and his/her members. Not all such relationships are friendly enough for a unilateral action like that to be welcomed by the members.
And I don't see a way to move archives over.I think this has to be done member by member, to avoid privacy and copyright concerns. And it probably needs to be both-ways voluntary. That is, it may be that the Groups.io group manager needs to designate which Yahoo or Google groups(s) may have content migrated into their Groups.io group. And then each member of the Groups.io group would have to elect to have their own content migrated from the designated group(s).
To make this as easy as possible for the member, the Groups.io manager would likely need to create a "migration" account in the target Yahoo or Google group, and maybe give it moderator access if needed to access the necessary metadata along with the member content (I think that would be necessary in a Yahoo Group, not sure about a Google group).
The alternative, asking the member to provide Groups.io with their Yahoo/Google sign-in credentials, fails utterly for "email only" members - those without an owning account - which happen to be the majority of my Yahoo group members.
Clearly, I think, ownership of the content must transfer correctly; and that I think is handled by having each Groups.io member authorize the transfer of his/her own content, identified as such by matching email address. That may require that Groups.io grow a mechanism for users to specify and validate "alternate" addresses that they own; in case they can't use or change the address used in the Yahoo or Google group.
I don't know what to say about the other metadata that goes with the content. Primarily that would be the date/time the content was contributed to the group - it would be nice if that was preserved. But there most likely a raft of other details for the devil to be hiding in.
What such a migration capability implies for sorting/searching the Groups.io content I don't know. Maybe there's a use for #tagging the migrated content to identify its original origin.