I think you either need to (1) get rid of this member function to keep ExcahngeNodeGroup as a struct force people to do `enodegrp.nodes.push_back(enode)` or (2) turn this into a class.  This is because it is not just about [setting up data members](http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml#Structs_vs._Classes) (as per GCSG) but rather about modifying existing data members.