Consider a social entity -- say the IBM corporation -- and consider the group of individuals who currently make up the entity. What is the relation between the social entity and the individuals? There are several things that are plainly true: the entity is composed of the individuals. The behavior of the entity supervenes upon the individuals. But other issues are less clear. Does the corporation have causal properties of its own? Is this corporation an instance of a broader class of social entities with similar properties? Do we need to explain the corporation by deriving its properties from the properties of its constituents?
There are analogous issues in other areas of the "special" sciences. Take weather, for example. We have a fairly complex vocabulary of weather ontology and causation: El Nino, warm front, high pressure cell, hurricane, windshear, etc. Two things seem evident. First, weather phenomena are composed of lower-level physical components: bodies of water at a range of temperatures, non-homogeneous gases (atmosphere), inputs of energy (sunlight), and the like. Weather phenomena are "nothing but" ensembles of micro-cells of gases, water, land masses, and energy. Weather states might be exhaustively described as a set of micro-states with no reference to weather vocabulary; we could, if we wanted, reduce weather statements to micro physical statements. At the same time, we can legitimately describe weather phenomena at the meso or macro levels -- storms, etc. There is no compelling scientific reason to insist on reduction or elimination. And we can explain weather outcomes in terms of causal statements that invoke other meso factors. The fact that there are no autonomous weather phenomena does not mean there are no autonomous weather explanations.
There isn't much of a problem in defining or identifying entities at the level of social aggregates. The IBM corporation is no more ontologically suspect than Hurricane Irene. Both are high-level entities composed of lower level things and processes. What is more problematic is when we consider whether there are "kinds" of social entities; whether there are law-like generalizations that are true of those kinds of things; and whether kinds of social things have distinctive causal properties and processes.
Candidate social kinds might include armies, bureaucracies, and religions. I take the view, however, that none of these concepts identifies a set of social entities that have enough in common to call them a kind. They are heterogeneous in all the ways that have made them intriguing to historical sociologists. Caesar's army, Rommel's army, and Ho Chi Minh's army had some things in common; but they also had enormously important differences at the level of organization, technology, and leadership structure. So it isn't plausible to look at them as constituting a kind of social entity.
This high degree of heterogeneity among the items classified under social concepts also provides the basis for a negative answer to the question about laws as well. Armies are not like metals; they don't have a common set of generating processes, and they don't give rise to significant regularities. We can't say things like "all armies fight to exhaustion within 12 months."
We can, however, identify common processes and challenges that confront all armies, and we can consequently tease out some causal mechanisms and processes that recur across armies in a wide range of contexts. This is the thrust of McAdam, Tarrow and Tilly's analysis of mechanisms underlying contention.
This discussion illustrates three rather different points. First, there is the level of descriptive concepts we choose to use. Second is the issue of whether explanations need to proceed from more fundamental to more complex. And third is the issue of the ontological status of composite entities.
These topics have been addressed under the rubric of inter-theoretic reduction for almost forty years. Jerry Fodor's "Special Sciences (Or: The Disunity of Science as a Working Hypothesis" (1974) (link) and "Special Sciences: Still Autonomous After all These Years" (1997) (link) present core arguments for the autonomy of the special sciences. He distinguishes between token-token identity and type-type identity, pointing out that it might be the case that the first kind of identity obtains while the second does not. His central argument is that the possibility of multiple functional realizability demonstrates that it will not be possible to reduce higher-level laws to lower-level laws.
Jaegwon Kim has addressed the issue of physicalism throughout his career, most recently in Physicalism, or Something Near Enough. He holds that the concept of supervenience provides a basis for finessing the demand for reduction from higher kevel to lower level. Julie Zahle raises some of these problems in her contribution to Philosophy of Anthropology and Sociology: A Volume in the Handbook of the Philosophy of Science Series, "Holism and Supervenience."
My general intuition is that the issue of inter-theoretic reduction and an insistence on a strong version of methodological individualism are much less important to the philosophy of social science than they have appeared to be. Once we have sufficiently understood the ways that individuals, institutions, networks, and values work at the local level, we are in a good position to characterize meso-level social entities like organizations and value systems. And we are intellectually empowered to try to discover the dynamic properties of these systems of actors and social arrangements. So meso level causal properties seem entirely legitimate in the social sciences.