Can karma operate at the level of groups i.e. collectively? In the book the Land of No Buddha the author interviews a Tibetan lama in which he talks about collective karma - to quote 


  All groups have karma that is more than just the collection of the
  karma of the individuals of the group. For example a group can decide
  collectively to start a war.


(pp 76 of above book)

This is the first and only time I have heard of collective karma. Is this a concept purely within Tibetan Buddhism? Is it wider than that or is it perhaps just this lama's understanding and it not really a concept in Buddhism generally? If it is a genuine concept then can someone give some more details and perhaps how it fits in with karma at an individual level?
Yes, of course, there can be group karma.

To begin with, individual karma is only a useful approximation. Because of anatta, what we conventionally call individual is merely an assembly point of contributing factors. It is these factors that are referred to as karma. They don't belong to anyone, really.

As per the Hierarchy Theory, useful notions of groups can be arbitrarily designated at any level of observation. Individual being, family, social group, country etc. -- all of these are abstractions. Some of the karmic patterns make better sense when looked from far away, in which case it may be helpful to attribute them to a group.