Question Admin GUI

Hey there! So i'm looking to create a little admin gui in chest commands. Basically what I want is to be able to do a command like /ag {username}. I have no problem making the effects an admin can do within the menu, but I want the username provided to be the player being affected. Ex: /ag badplayer1, admin clicks freeze player in the menu, then badplayer1 becomes frozen. Does this make sense? Any suggestions?