It looks as if a single representative of a choir has got to be prepared to register on behalf of the whole choir – is that right?
Yes, that is the basis for the way the site has been designed, due primarily to the way in which rights-owned music is sold. The purchase itself is for a number of licences to make copies of the digital product so the purchase needs to be a single transaction on behalf of the choir in question (e.g. the purchase of 50 copies of a sheet music arrangement bestows the purchaser the right to use up to 50 copies of that arrangement with their choir)