do JW say grace and how
Some I have known do, some don't. One former brother-in-law did so in the company cafeteria, but silently: you wouldn't know if he was JW, Protestant, Catholic, Muslim or Jewish, only that he was praying silently.
When said aloud, such as at congregation "get-togethers" or big family dinners, I noticed that the prayer before meals was only partly about giving thanks for "our daily bread", and always included a long mention of the congregation, etc. Also, no Witness I have known called it "grace": that's too much like what "worldly" people following "christendom" do. It was "Will brother Smith lead us in prayer", never "Will sister Jones please say grace". The genders in those statements was deliberate; I don't recall a woman witness ever leading a prayer.
BTW, they usually won't say "JW". They refer to themselves "Witnesses", "the brothers", "the sisters", "the friends", "the truth", "the society", etc.: insiders' language.