How can I add in commandbutton inside datatable?

<hx:dataTableEx value="#{searchData.searchFriends}" var="s">
   <hx:columnEx>
    <f:facet name="header">
     <h:outputText value="First Name" />
    </f:facet>
    <hx:requestLink action="#{pc_Search.doAddFriendAction}">
     <h:outputText value="Add as Friend" />
     <f:param name="friendId" value="#{s.memberId}" />
    </hx:requestLink>
   </hx:columnEx>
  </hx:dataTableEx>

To get the data at backend

String friendId = (String)getRequestParam().get("friendId");

But once I change the requestlink to command button the friedId = null? any idea how can i pass value using command button