Providing website users with appropriate capabilities and access is a powerful built-in feature of WordPress. It is an important and helpful tool for your users as well as the administrator(s). It is important to get familiar with the basic roles that are always there, ready to use. Go ahead and check out the overview provided by the WordPress codex.
An enjoyable and extremely useful read if you want to get familiar with usability testing. Everybody working with web sites ought to have read this book – not only web designers and developers but web site owners as well.