Telepathic humming

If appropriate, ask singers to close their eyes.

Singers each pick a random note and all hum together simultaneously (mp), or sing a very gentle ah, creating a random chord. Take a breath whenever they need to, by fading out and fading in again, so the overall chord doesn't get interrupted. (Can talk about staggered breathing here.)

Ask singers to gradually turn their attention to the other notes that are going on around them, specifically their neighbours to left and right. Do this without stopping their own note, and without moving their heads - do it simply by switching their mental focus from one nearby note to another. The aim is to become aware of specifically which other notes are going on around them.

Now ask singers to continue humming, but gradually coalesce onto a single unison note. Don't specify which note - just invite them to listen to what's happening around them and gradually agree as a choir on a note. There's no rush. Depending on the size of the group (and the stubbornness of individuals!) this might take a couple of minutes. If the group reaches a deadlock on two or more competing notes, resist the urge to intervene - let them settle it. Emphasise they are to do it by 'telepathy'.

Still without interrupting the continuously hummed note, and without giving any direction, develop as appropriate by:
 * changing the unison to a major (minor / diminished / whatever) triad
 * changing back again, and trying to get the group to produce a cleanly-timed change without any conducting
 * precisely tuning and balancing the note/chord by listening to the ensemble
 * adding a 7th to a major triad, then asking the group to resolve it
 * shifting the current note/chord up by one semitone, again as neatly as possible without conducting

To finish, ask singers to fade out as smoothly as possible to absolutely nothing — again telepathically, without any conducting.

In a suitable acoustic, the whole exercise can be done with the group standing in a circle and facing outwards — this forces them to listen to the ensemble sound rather than the sound of their neighbours.