Group psychology focuses on how groups come to be, how their identities are forged and shaped, how they grow, and ultimately even what they come to do e.g. if they tend towards violence or arm theirselves why do they do so?
Given that these questions are so wide ranging the subject obviously finds ground in various other subjects. Yet for example in International Relations there is a lot more research needing to be done. The main theoretical branches of IR use almost no psychological justification, and yet all are willing to base their ideas upon very profound psychological components of human nature. Realists cite power and security as the main motivation for group actions, Liberals cite utility or wealth maximization, and constructivists call human nature nothing more than a social construction.
Do any of these views hold true to you? Or would you need to see the data? Why do you join and/or start groups? And if there could be one then what do you think is the primary motivation behind group actions?