Create a jetty-web.xml file under your war/WEB-INF folder
Mine is defined as follows
<?xml version="1.0"?>
<!DOCTYPE Configure PUBLIC "-//Mort Bay Consulting//DTD Configure//EN"
<Configure class="org.mortbay.jetty.webapp.WebAppContext">
<Get name="securityHandler">
<Set name="userRealm">
<New class="org.mortbay.jetty.security.HashUserRealm">
<Set name="name">default</Set>
<Set name="config">c:\ocs\jettyusers.properties</Set>
</New>
</Set>
</Get>
</Configure>
my jettyusers.properties file contains the username, password and roles for each user e.g.
pstockley: test, nirvana_tester