Group posts by author’s role

Might see if this helps:
https://wpengineer.com/list-all-users-in-wordpress/

I gave up and decided to set up a parallel database in a subdirectory.

You can do the following:

1) Restrict users to certain roles/capabilities using Role Manager
2) Assign a parent category to front page posts
3) Force certain users to that category using Level2Categories
4) Hide all but that category from the front page using Advanced Category Excluder

You could then use something like List Category Posts to list other categories on a separate page, or hard-code it. (If anyone knows of a more robust plugin to display posts from a certain category on other than the front page, let me know.)