Parent groups on DUG are our "root" user groups, which exists for all of the major Microsoft Dynamics products: Dynamics 365 for Finance and Operations/Dynamics AX, Dynamics 365 Business Central/Dynamics NAV, Dynamics 365 for Sales/CRM and Dynamics GP.
It's inside these groups that you find the members and our question/discussion forums. Plus all the blogs and other resources (downloads, documentation, events, videos, wikis etc.) for our group members. If you want to participate, then you need to join at least one of these groups.
Then we have an additional Dynamics 365 parent users group. This is for all users of Dynamics 365, especially covering wider D365 topics, not already covered by our other groups.
If Microsoft would add more products to the Dynamics 365 family and our members are requesting it, then new parent groups could be started.