I didn't see this in the rules, but can a 3rd party "support" a battled area? Like in A Game of Thrones. Picture if you will, letter "B" as the center of a 5 square group of adjacent areas.
A - B - C
Blue controls "B", "E", and "A"
Red controls "C"
Green moves into "B" to attack. Can Red in area "C" choose to add a die per gang member to the fight in support of Blue or Green??