Game situation: 4 seconds left in the 4th quarter. Home team ahead by 2 points. Visitors are attempting a field goal. Field goal is blocked and R picks up the ball and runs the length of the field for a touchdown. During the run, time expires and nonplayers from R's bench come onto the field.

What is the proper enforcement?

It doesn't matter. Non-player fouls are penalized from the succeeding spot. Count the TD and administer USC if you like but basically, the game is over unless you have to run the try.

I agree. Since in this situation the point after will have no affect on the game, I'd say game over.

If nonplayers only come onto the field then your answer is above. But if nonplayers affect the play while the ball is being returned then you have illegal participation penalized from that spot. Play one untimed down.

We had a similar situation this year in a JV game. Fumble recovered and returned for about 30 yards, not a TD. Players from the returning team's bench swarmed on the field during the play. We penalized unsportsmanlike conduct for more than 11 on the field. Penalize from succeeding spot. 1st and 10. Had they participated in the play, it would have been illegal participation.