Initially I added a user in the $catalina_home/conf/tomcat-users.xml
Then I add the security constraint in the web.xml of the application. In my case it was the default application ROOT. Hence I added the following entries in $catalina_home/webapps/ROOT/WEB-INF/web.xml
The role that is required to log in to the Manager Application
Hence all the users under the security role "agroup" will be able to access the applicationn.
Hope this helps some one somewhere..