Several times I would like to have been allowed to warn others in my vicinity of "chinook at 11 O'clock climbing towards" or "gliders circling ahead" or even "kinnell, that was close, where is the other one" (fast jets always fly in pairs!) but it seems that we shouldn't.
Nothing to stop you requesting this info be passed on by the controller, " XXXX, please advise G-XX of Chinook at 11 O'Clock, heading our way" as your other aircraft will be on the frequency anyway they will get the info as soon as you transmit it.