With chapters on cooperation, social institutions, cultural evolution, and group responsibility, this text systematically studies social groups, acting in them as a group member, collective commitment, group intentions, beliefs, and actions, especially authority-based group attitudes and actions.
Possibly the driest, most technical and seemingly repetitive book I've ever read. Unless you really relish plowing through formal definitions, endless lists of criteria and subcriteria, or you need to read this for your research (which was the case for me), don't even start to read this. Tuomela uses examples sparingly and when he does so, usually he only devotes one sentence to spelling out the example. Sadly, this makes his discussion frustratingly abstract and difficult to understand.
Of course, all of this applies only to the style in which Tuomela presents and elaborates his we-mode account of group action. It does not in any way detract from the plausibility of it. I just wish that he would have included a 1-2 page wrap up of his account. Perhaps what he writes in the introduction qualifies as such. Even so, I do think he uses too many different concepts that barely seem distinct from each other, to explain his account. Intentions, commitments, the Collectivity requirement, ethos, etc., somehow all of these concepts play an explanatory role but after plowing through 100 page of technical analysis, you lose track of what is grounded on what and which specific aspect of the we-mode account they are supposed to explain.