I only have experience with this once, and it was with United Airlines (I know, I know, it was a low moment and I was not in charge). In this case, any changes had to go through the group ticketing office and needed to be made by the person who was listed as the group leader. Once I knew what name that was, I was able to call in and say that I was that person and make any changes I wanted to, but it needed to be done by group sales. You might try to a conference call to group sales with that person if they are willing.