Somthing that should be added, you can add document groups and user groups to any document and/or folder, HOWEVER, if you want the documents to show up in the tree on the left side of the manager, the user MUST have access to the toplevel document, look at the following document tree... ("+" means folder, "-" means document).
ROOT
+ Blogs
| + Categories
| | - MODx related
| | - Random
| - Tags
If you give a user access to the "Categories" folder, but NOT to the "Blogs" folder, the user will not see any of these documents in the tree (as of version 0.9.5 at least). So the folder access must travel up to the top (root) of the tree. It is possible though to give users access to the "Blogs" and "Tags" but no access to "Categories".
It’s a little confusing since this behavior was not what I expected.