Wordpress CiviCRM groups, members and permissions
I can answer some of these questions:


CiviCRM doesn't store usernames/passwords on its own.  It integrates with the Wordpress authentication system
You create user roles in Wordpress' User Role Editor.  You can then go into CiviCRM and grant CiviCRM permissions based on Wordpress role.
Because you say you want users to "upload via the frontend" and mention that they have logins - I assume you mean that these are users who have permissioned access to Wordpress, but not CiviCRM.


With CiviCRM, you should:


Enable a user dashboard (to allow contacts to edit their own contacts);
Create a search profile (to enable members to search for other members);
Create ACLs (to allow you to restrict access to contacts based on their committee membership);
Create several document submission profiles, to allow folks to submit/search documents that they're supposed to be able to see.  I imagine I'd create a CiviCRM contact for each committee, then create a profile submission form that allows permissioned users to add activities with attachments to that user.


I could also see a different approach where the documents are stored in Wordpress and permissioned that way, but I don't know enough about Wordpress to know if that's feasible.  I'm thinking of a feature similar to Drupal's Organic Groups.