Hey, jumping in with my .02
- Circles are great, because users will just figure them out and not need admin rights to create and manage them.
- Group Folders app is too buggy, so I recommend just carefully laying out a folder system that makes sense to your group so everyone can benefit from Trash, Version Control, Workflow Automation + File Access Control app. Adding Retention policies is also helpful if you are concerned with random files clogging up specific folders over time. I found these apps were all useful for my organization.
Right now these systems address different issues, but will not integrate. 
- Projects will group files and folders to Deck cards and Talk conversations. It is part of the core Files app.
- No other functionality supported yet. Will not display comments, does not support Activity feed, cannot be assigned to circles or groups. … what you see is what you get.
Discussed on the forum with lots of links here.
- Circles allow users to create their own ad-hoc groups. Instead of permissions I can simply create and control my own circle of sharing.
- Federation to remote users simply doesn’t work… I’ve tried over and over, but something is busted.
In-depth review of Circles app from a year or so. The app is almost in the exact same state now.
- Group Folders is an app that allows admins to create all their groups + set specific permissions from one admin page. Great idea, but problematic implementation
- This app has a lot of problems. Honestly, it is garbage since it is so broken. Be very careful as even deleting the app completely will still cause problems if you attempt to use the same folder names again. I found I had to delete Nextcloud itself rather than just the app.
Hope these links help you!