On the Admin page, there is a link under User Management that can restrict the system to only show records assigned to the user. This is a global flag so it will restrict Accounts, Contacts and Leads as well as documents.
If you need more granular control, our Professional edition has a team management feature that allows you to assign people to teams and teams to items.
There is a MassUpdate configuration flag on each module. Just uncheck the flag in the Manage Modules administration page.