Max Abrahms' paper "What Terrorists Really Want" claims that belonging is more important to recruitment than ideology, and that terrorist groups are not fundamentally concerned with achieving political goals. Terrorists join because they know people already involved.
The claims may be largely true, but don't address the formation of such groups. Keeping tabs on all the acquaintances of suspected terrorists is undeniably useful, but not sufficient. Somebody starts these groups. Are they founded by sociopaths who latch on to some ideology, or by the "rational actors" trying to achieve particular ends? The initial group must form with a different social dynamic than the ones he says bring in later recruits. I think this analysis is incomplete.
Pointed out by the Assistant Village Idiot