Binary Image, Concentrations of White Dots with OpenCV

Given a binary image of black, with a few sporadic white dots, I'm looking for a way to "lasso" a majority of the white dots inside a bounding rectangle. Consider this image:

See how not all of the dots are enclosed, but just the clusters (or set of clusters) where there are clearly more white dots than anywhere else?

I already know how to put a bounding box around all of the white dots with OpenCV. Can anyone direct me as to how I can analyze this image for one big concentration of dots, ignoring any peripheral dots which are not really part of a group?

N.B.: This bounding box doesn't have to be skew. Even a convex hull would be nice as output.

I can adjust the size, color, alpha, location, density, etc. of these dots. So if your thoughts involve doing something with these dots in order to process them, that might work.