In Chapter 4 of The Chrysalids, who is the group?

David Strum is able to communicate through some sort of extra-sensory perception with a limited number of people, including his half-cousin, Rosamond Morton, and a group of others.  When David's Uncle Axel discovers David and Rosamond's ability, he warns David to never reveal this means of communication to anyone. If anyone were to find out, David and Rosamond would be considered to be mutants, which would mean they would be killed or run out of the community.  Uncle Axel has no idea that there is an entire group of such people, and David does not share this information with him.  But after Uncle Axel's discovery and warning, David passes this on to the rest of the group, and it is at this point that they do become a cohesive group, rather than individuals associated only because of their mutant ability to communicate.  They all promise to keep their ability a secret, to protect themselves and one another.  David says,

It was our first act as a group; it made us a group by its formal admission of our responsibilities towards one another (32).

He feels that this first step not only helps them to protect them but also comforts them, giving them "the feeling of sharing..." (32). As the story goes on, this group features more and more prominently, finally culminating in the group being possibly the last best hope to save the world from itself.