https://help.yahoo.com/kb/groups/SLN31010.html?impressions=true
Another option for migrating is Discourse. Jeff Atwood, one of the two founders of Stack Overflow, decided to try reimagining forum software the way he and Joel Spolsky had reimagined Q&A sites. Discourse adds badges and reputation to forum software in an attempt to guide users into forming a healthy community.
Here's a relevant blog on how some video game communities use Discourse.
I've poked around on Discourse, but I've never been a regular user. I've spent a ton of time on Stack Overflow, though, so I like the general philosophy. (It's let me help over six million strangers on the internet, so I think it's been time well spent with effective tools.
https://stackoverflow.com/users/4794/don-kirkby )
As for cost and migration, they'll set up a Digital Ocean site for $100, then you're in charge of maintaining the software and paying $5 a month to Digital Ocean. The software is open source, so if anyone in our community wants to host it, there would be no charge.
There is a script to migrate from Yahoo! Groups into Discourse:
Don