Ok...
First they can't use Yahweh's Witnesses since another group uses that name I believe. There are MANY denominations of christianity that use Yahweh or Jehovah.
I have a bible on my shelf that has Yahweh in the NT more the the NWT does!
As far as to how YHWH is pronounced a poster above pointed out various Hebrew names contain YHWH so a general idea amoung schoolars seems to be Yahweh.
Of course the Shashu pople who orginated thos god would be rolling on the floor laughing if they knew 3000 years later people were debating or even still worshipping their god.