R. v. Powell, 2021 ONCA 271, April 29, 2021, at paragraphs 50 and 51:

…mere participation in a melee in which the complainant suffered bodily harm will not support a conviction of assault causing bodily harm unless the accused person had the requisite mens rea. The accused person’s own act of participation must be intentional as opposed to accidental. Moreover, before he can be held responsible for bodily harm that may have been caused by another person involved in the group assault, it must be found that the accused knew that the others were engaging in an assault, and intended to assist in that assault.

…an accused who is involved in an intentional assault that results in bodily harm cannot argue that he only intended the assault and not the bodily harm that resulted, provided that a reasonable person, in the circumstances, would realize that the force intentionally applied would put the victim at risk of suffering some kind of bodily harm: R. v. Palombi, 2007 ONCA 486, 222 C.C.C. (3d) 528, at paras. 38-39. This principle applies equally in group assaults. Where a reasonable person would realize that the group assault would put the victim at risk of suffering some kind of bodily harm, an accused person who has joined in a group assault cannot avoid responsibility by arguing that they did not intend to cause bodily harm.