OK quick question.If i have a unit or multiple units on standby and my opponent charges one of those units can I use my standby token to shoot them?

If the enemy troops move into melee (trooper vs trooper) with one of your units, you can activate your stand by token on every unit in range 2 that has LoS on them. But only AFTER the enemy move action, since then they are already in melee (because standby activates AFTER the move / attack action but BEFORE additional keywords like relentless or charge) you cannot shoot them BUT you can melee attack them even BEFORE they use a possible Charge keyword to attack you after their move.

Hope that clarifies it for you ^^

tl,dr: You cannot shoot them, but you can melee attack them