To remove a user from your group, navigate to their profile. You can do so through the Members tab or from a challenge.
Once in the user's profile, click on the pencil icon in the upper right-hand corner.
In the pop-up screen, select "Remove from group".
Refresh your browser to see changes take place immediately.
Users removed from a group will no longer appear in the Members tab, and will be removed from all pending and started challenges of yours. In order to rejoin your group, they will have to take the same steps as they did when they joined your first challenge. The act of following a challenge link associated with your client will re-associate them with your group.