Thank you Rob -- I stopped reading when it said "New in 2.9" and didn't realize there was a footnote beneath.
For anyone else that may be following this thread, I copied the text below:
Single simple discussions cannot be set to separate groups (as this feature is not implemented). Instead, a standard forum should be used. If required, a permissions override may be set to prevent students from starting new discussions.
My problem with all the other forums is that none show the fully expanded discussions on the main page except for the Single Simple Discussion (for obvious reasons). But I will do as it suggests so I can use my groups. Thanks for your prompt help : )