The initial attack has 100% accuracy and ignores Reaction Abilities. Other characters, with a combo set, may join in if they are within their range (some can join in up to five spaces away, others have to be right next to the victim).
Each joining unit has a specific range, accuracy, and percent of damage added, when adding to a combo. Combos with more range or damage added are usually less accurate.
If one or more members successfully adds in, then extra Judge Points further increases damage.