Hi roxy1987,
What version of JBPM do you use?
in JBPM5.4, Human Task Service will use UserGroupCallback to get the user's group, so when the userGroupCallback component cannot prove that mary belong to the group in which the task is assigned, you will get that error.
You can give mary membership to the group by editing the roles.properties file or you can override and write your own implementation of userGroupCallback if you get your user and group relation on other place such as database.
if you write your own implementation of userGroupCallback, make sure you tell HumanTaskService to use your component by specifying it on
\jbpm-human-task-war.war\WEB-INF\web.xml
<param-name>user.group.callback.class</param-name>
<param-value>org.jbpm.task.identity.DefaultUserGroupCallbackImpl</param-value>
Regards,
Thomas Setiabudi