Get persion form a group

[evan]

Hi All;
     I know workflow can be used foreachfork to get all member in one group.
 

   <action class="org.alfresco.repo.workflow.jbpm.ForEachFork">      <foreach>#{people.getMembers(wf_reviewGroup)}</foreach>      <var>reviewer</var>   </action>  ‍‍‍‍‍‍

  now i know that I can use the following code to assign task to all member which is in the group.

 <assignment class="org.alfresco.repo.workflow.jbpm.AlfrescoAssignment">        <actor>#{reviewer}</actor>      </assignment>‍‍‍‍‍

My question is ,if I want to assign task to a specified person who is in the group.How can I get the person ?

[evan]

Help me ! who can teach me ? There is one user was already assigned two groups,but I don't know this user . so ,I want to search this common user from this two groups in workflow . and put user id in swimlane . how can I do  this?