In Chapter 9 of The Chrysalids, how is Michael shown to be the leader of the group of telepaths?

Are you sure you mean Chapter Nine? I have just re-read it
and Michael does not appear at all, apart from references made to "the others." I wonder
if you actually mean Chapter Ten. This is the first stage in the novel where the
character of Michael begins to show its leadership, all triggered off by the surprise
announcement that Anne is intending to marry. We are told interestingly that out of the
group, it was Michael who was the first to respond to Anne and the first to express the
thoughts, feelings and fear of the wider group. Note what he
says:

"You
can't, Anne. For your own sake you mustn't," he told her. "It'd be like tying yourself
for life to a cripple. Do think, Anne, do really think what it is going to
mean."

It is he who opts to
say these rather blunt words to Anne, rather than anybody else, clearly indicating the
future role he will play in the group as leader, which becomes more pronounced as time
goes by.