Unlikely, he wasn't supervised by a parent, guardian or authorized (by the parent/guardian) adult. That might apply when he was with the group, but likely not when he went off on his own.
See my post above. The actual Wisconsin law, which I provided links to, does not require anyone to "accompany" a 17 year old. The age limit is actually 16.
Moreover, even if the "accompany" requirement did apply to a 17 year old the act of accompanying someone does not require that the people be together at all times. Think about the ordinary use of the term. And think about what happens when a father and his son go hunting together. They may well be separated at times.
Under the normal use of the word "accompany" one person can go off by themselves and then return. If you accompany your wife to the theater would that be untrue if she went to the ladies room? Of course not.
This is the elephant in the room that can't be touched, here or anywhere else.
While each shooting might be a legitimate act of self-defense, if Rittenhouse left the group at the boarded-up business on his own, then the whole portrayal of him as a protector goes right out the window; he set himself up for trouble, and found it in spades.