Having a google around, there aren't really any consistent lists of signals for riding in groups. I'm more interested in what people use in relatively slow social groups, rather than racers, but if there is anything specifically used, it would still be nice to know.
Obviously there are the traditional left and right turn hand signals, pretty much used around the world (and not just in groups).
Also fairly consistent is pointing down at the ground on the left or right for debris/obstructions, often with a cry of glass/hole/etc.
There is also a sign sometime used where people put an arm (typically the left) behind their back, pointing (again for a left hand, rightward) indicating that they are about to pull across, normally to overtake a parked vehicle, or something of that ilk. I haven't really seen this described anywhere.
Braking is normally indicated by a cry of "slowing" or "stopping", although very occasionally people will use their right arm in an up and down motion. I've generally only used this one to slow car drivers down, when they are approaching the group, and I'm either at the front or rear enough that this warning will have some effect before they get to the group. I suspect this is as much because they are unsure what I'm signaling for, as much as anything else.
The calls of "Car up" or "Car down" are also fairly normal, along with various other related cries, such as "Bicycle down" on yesterdays Ride to Regicide, where we were on a fairly narrow riverside path.
There are also various random cries, such as "bollards", "loose gravel" etc
