Ok I think I got it now...
It doesn't matter who harvests because it's the grower who gets the belt increase and the extra bonus crops.
There is no current list that shows how many crops need to be harvested to reach each belt level. (That's ok, I can live with that)
To see how many harvests I need to reach the next belt level I can hover over the crop in the shop. (Done that, it works).
And the best one which I am following now is to make a list of friends called "farmtown" and post the bonus offers, if I choose to post at all, to that group only.
Problem solved. I'm off to grow some more crops and aim for the highest belts. And being the one that likes things evenly done, I will grow stuff I don't need, just to get the belts for that one too. Why? Because I'm mad. LMHO. Happy Farming everyone!