Complex sentence: End-user consumers can, in theory, be non-participants of the MLM company, although in practice they are overwhelmingly themselves participants (aka "salespeople", etc) of the MLM who had been earlier recruited by a fellow participant above themselves in the pyramid.
Split into simpler sentences: End-user consumers can, in theory, be non-participants of the MLM company. In actual practice, however, they are overwhelmingly themselves participants (aka the very "salespeople", etc) of the MLM company who had been earlier recruited by a fellow participant above themselves in the pyramid.