D.H.J. Polymath is the assumed collective pseudonym for the authors of a number of papers which have arisen as a result of the polymath project initated by Gowers. Presumably, since it is a matter of open record, one can go through and identify the participants and their various individual contributions.
But what is it that mathematicians on the street think when they answer this question? The answer is that D.H.J. Polymath is equivalent to T.Gowers, T.Tao, et. al. I don’t claim that this is an accurate reflection of the contribution of the participants, but simply the perception in the community (as far as I have gauged through a number of conversations). Credit for joint publications can be a tricky issue at the best of times. In this case, the people responsible for making the decision to choose a pseudonym are presumably those for whom receiving credit is the least relevant. I find myself sympathetic to the remarks made by T.Brown in the mathscinet review of the first polymath paper:
This reviewer would have preferred to see, rather than the pseudonym “Polymath”, a list of authors. In other fields there are papers with a hundred co-authors. Why not in mathematics a paper with twenty or thirty co-authors, with extra credit for the person(s) who wrote the exposition?