1. I can see a lot of overlap with this and several senses of the the term institution. The reason I find it convenient to use a different term is that it shifts the emphasis to what specific groups are doing. For example, family is an institution, but the Templeton family is a unit of action. The corporation is an institution, but IBM is a unit of action. It also usefully excludes broader institutions, like the market or civil rights, while keeping the New York Stock Exchange and the ACLU as units of action. One way to think of it: units of action are the microphenomena of institutions, and the macrophenomena of people.
2. Coming from a firmly demographic perspective on groups, like is common in political campaigning, this could easily get fuzzy. Consider religion: Christian and Muslim are not units of action, but Mormons and Catholics are essentially big hierarchies while Sunnis, Jews and Evangelicals are not. In the campaign-view of groups, what I think of as units of action are mostly important because they are indicators for demographic groups: NAACP is an indicator of black voter support, AARP is an indicator of senior voter support, unions of working class voter support, etc. This view seems to get the most airtime by far, though I could be biased because I consume an unusually high amount of political information.
Caveats
1. I can see a lot of overlap with this and several senses of the the term institution. The reason I find it convenient to use a different term is that it shifts the emphasis to what specific groups are doing. For example, family is an institution, but the Templeton family is a unit of action. The corporation is an institution, but IBM is a unit of action. It also usefully excludes broader institutions, like the market or civil rights, while keeping the New York Stock Exchange and the ACLU as units of action. One way to think of it: units of action are the microphenomena of institutions, and the macrophenomena of people.
2. Coming from a firmly demographic perspective on groups, like is common in political campaigning, this could easily get fuzzy. Consider religion: Christian and Muslim are not units of action, but Mormons and Catholics are essentially big hierarchies while Sunnis, Jews and Evangelicals are not. In the campaign-view of groups, what I think of as units of action are mostly important because they are indicators for demographic groups: NAACP is an indicator of black voter support, AARP is an indicator of senior voter support, unions of working class voter support, etc. This view seems to get the most airtime by far, though I could be biased because I consume an unusually high amount of political information.