Liability of the management party

Hello, We need an urgent advice regarding the following case. there was a music concert organised by a private limited company, say XYZ and asked group of few people, say GRP, to manage the concert. the group of people is not any constituted entity its just a group of few friends who accepted the proposal to manage the event. while conducting deals with the parties for the concert the GRP dealt with some vendors as per the directions of the XYZ however the concert occurred loss and all the vendors accepted the payment as much paid by the XYZ leaving some of the payment except one vendor who does not want to settle at lower payment and forcing GRP the settle the remaining payment. Even after so many requests from GRP to the vendor he never came with them to meet XYZ to settle the claim and stuck at excuse that he dealt with GRP so he shall claim money from GRP. other fact of the case are there is no written agreement ot evident agreement that there was any deal between XYZ and GRP except on the banners and publicity documents XYZ was shown as "Organiser" and GRP as "Managed by" there is no written agreement or any evident agreement between GRP and the vendors dealt with kindly guide whether GRP is liable to any vendor in case of loss where they were acting as a person who were managing the event in case of loss