In primefaces you can use code like below :
<p:selectOneMenu value="#{removeAgent.groupNumber}" effect="drop">
<f:selectItems id="resGrp" value="#{removeAgent.allGroups}" var="grp" itemLabel="#{grp.groupName}" itemValue="#{grp.id}"/>
<p:ajax update="nextElementID" listener="#{removeAgent.valueChange}" oncomplete="submit()" />
</p:selectOneMenu>
update attribute of ajax tag is DOM element that should be updated after the ajax request. It's very useful if you have select form like city/province and so on. If you don't need, you can leave it out.