A coach (any associated with the high school) can only coach/train 8 returning varsity players per team in the offseason. The eight returning varsity players include any player that dressed for a varsity match the prior HS season. In other words, the player didn't even have to play, simply dress to be considered a varsity player.
There are many cases where high school coaches (head coaches, assistant coaches, volunteer coaches) double in the fall as club coaches. As long as they don't exceed the 8 player rule, they are fine.
Not sure on the "open training" example you gave, but my understanding is that the rule pertains to coaching a "team."