How does Jack Merridew show that he has power within the choir?
From the very beginning of the novel, Jack demonstrates his control of the choir. When they first appear, they are marching down the beach, in full uniform despite the horrible heat and having just survived a plane crash. This is driven entirely by Jack and his control over the choir. When they go to elect a leader, the choir and some other boys vote for Jack and it is clear that Jack isn't going to stop.
Even the way that Jack speaks to the choir and labels them as "my choir" indicates the control he has over them. He isn't willing to go along with Ralph's plan unless he can control his choir and have them be the hunters. He then goes on to expand his power by making it clear that his way is the exciting way and the one that will let the kids eat meat and hunt and be free of the responsibility of keeping a fire lit, etc.