honestly they start with friends and family and the blizzard in house people. they test things that are on the queue to test but that only goes so far.
they add in streamers and a random assortment of lottery winners who test play many different things at once due to the random focus they have laid out .
some test questing. some test dungeons. some test farming ore and herbs and skinns and even motes and primals.
some like me test the starting areas for the draenei and bloodelves cause thats what my focus and want to do is.
the random people being random in their focus permeate through the game and report bugs in all facets of the game.
its truly a lotto after the targeted invites.
and that is perfectly fine as a way of testing.