So you've got a range of people who will be editing the website and you want to let some of them post content and use the file manager to do so, but not get at the rest of the dashboard? Great! Very doable with concrete5.
1) Turn on advanced permissions. You might be able to do this without, but you're going to want them anyway.
2) Create a group for these people, lets call it "editors"
3) Give the "editors" group access to the pages in the Dashboard. If you haven't figured this out yet, go to the sitemap as admin, and click Advanced and turn on the Show System Pages flag. Now the sitemap will include a Dashboard node that has pages for every dashboard screen right there in the tree! That's right we use concrete5 to power the concrete5 dashboard. You should be able to go into the file manager and add the editors group to the view permission for the file manager page.
4) Go find the File Permissions page under settings (you can always search in the toolbar for "permissions" and it will come up.) You will find an "Search File Folder" task permission that you ALSO need to add the editors group to. This is a common oversight and while admittedly easy to miss is required for this to work.
Clear cache and now your site editors should be able to bring up the file manager whenever a block or page edit mode requires it.