There are lots of people looking for help due to the upcoming changes at YahooGroups.
This group (GMF) has a wiki that everyone, member or not, can use.
The section for group members includes a Quick-start guide. How to join. How to log in. Reading, replying through email and on the group homepage.
And much more.
There is a large section with Best Practices for Group Management.
The first topic is an overview - Group Formation. Advice about subgroups, member management, bouncing, etc.
Topics reflect frequently asked questions so I hope you will find the wiki helpful for the transition. You can also suggest that potential or future group members can look it over. Just use the URL.
Additionally there is the official help - https://groups.io/static/help
Of course, you can ask here or search the Message archive.
Just another happy user of Groups.io
More help: use the search button at the top of the Messages list.