The idea from this paper by Ronald Burt is that people who participate in a number of groups or networks are exposed to a wide variety of perspectives and ways of thinking.
By virtue of this exposure, these same people are able see holes across groups and find new ways (new ideas) to bridge the holes. This often results in innovation.
I've only read about 1/2 the 58 page .pdf, but I highly recommend it.
Clay at Many-to-Many has a little more on it:Many-to-Many: Social Origin of Good Ideas