from fm_mission_controller.c4 (variable names changed for clarity):
int groupID = PLAYER::GET_PLAYER_GROUP(PLAYER::PLAYER_ID());
PED::GET_GROUP_SIZE(group, &unused, &groupSize);
if (groupSize >= 1) {
. . . . for (int memberNumber = 0; memberNumber < groupSize; memberNumber++) {
. . . . . . . . Ped ped1 = PED::GET_PED_AS_GROUP_MEMBER(groupID, memberNumber);
. . . . . . . . //and so on