Notes on Nationalism is quite different from 1984, more focused on its effects on the individual (obsession, self-deception, etc.) than tyranny per se. He makes the distinction between nationalism and patriotism and his use of term is a bit unconventional (for example he includes pacifist rhetoric as an example of what he calls nationalism).
I think defining the nation as simply "the decisions of a group" loses a lot of subtlety. E.g. what separates the term nation from country or corporation? Drawing on Benedict Anderson's Imagined Communities, I think identity, community, and mass communication play a big part in this.
An interesting contemporary example of the development of nationhood, was posted here a while back about the transformation of Taiwanese identity, from majority Chinese in the 80s to majority Taiwanese in the 10s.