I've more often seen collectivism as an opposite to individualism: that is, a desire or tendency to work together in groups or to focus on the welfare of a group more than that of individuals in it, or in some cases a focus on preventing harm over maximizing freedom. Tribalism is better because it doesn't have any other meaning. Also, the tendency Orwell is talking about probably evolved during conflicts among hunter-gatherer tribes, but I have no citation for that.
I have found this most wonderful (if fairly lengthy) article, and thought you would enjoy having it brought to your attention.
It is so good, I think we should include it among the references in the "Politics is the Mind Killer" wiki page. But, before that, I submit it to you, and ask you: is there anything in this article that would warrant its exclusion from this site? I mean, besides the fact that it is about politics, and written by a notorious Social-democrat (And is that in itself grounds for exclusion?).