Individual warding and group warding does not seem to stack:
A yeti with 30 personal warding and a L1 ranger with 10 group warding attacked a neutral wolf, which in V0.9 has 16 ambush again (beware neutral ambushers) The battle info displayed a limit of 11 for the wolf to ambush. The rules for warding were outlined by piranha.
Now 100-(30+10) would be 60% and 0.6*0.16= would be 9.6 -> 9
(since everywhere else, warbarons seems to truncate instead of rounding).
Otherwise 100-30 = 70 and 0.7*0.16 = 11.2 -> 11
So I deduce that group warding does not stack with individual warding. Is this a bug, or is it a feature?! If it is a feature, it would be inconsistent with they way other bonuses are stacking (or used to stack in V0.8).
PS: I actually just tested it. Attacking with a yeti on its own also displayed the wolf's ambush at 11, so it does not stack.