DataModel must implement org.primefaces.model.SelectableDataModel when selection is enabled.

just add this attribute rowKey to the datatable tag :

<p:dataTable border="1" value="#{projectAdminisrationMB.projectNoUsersList}" 
 var="userObj"
 rowKey="#{userObj.name}"selection="#{projectAdminisrationMB.selectedUsers}"
 selectionMode="multiple" rowIndexVar="rowIndex"
 binding="#{table2}">