I think what you need is a h:selectManyCheckbox, h:selectManyListbox or h:selectManyMenu, anything that allows you to select many groups at same time to associate them with your user. Try this:
<h:outputLabel value="#{bundle.CreateUsersLabel_groups}" for="groupsCollection" />
<h:selectManyMenu id="groupsCollection"
value="#{usersController.selected.groupsCollection}"
title="#{bundle.CreateUsersTitle_groups}" converter="omnifaces.SelectItemsConverter">
<f:selectItems value="#{groupsController.itemsAvailableSelectOne}" />
</h:selectManyMenu>