Custom Skins on my GMod Server

Tried some googling but couldnt find anything on this: How would I go about using custom skins i installed to the server but only allowing certain people to use these custom skins, for example lets just say i want all admins to look like Shepard from mass effect and i have all the player model files installed onto the server how would i allow them to look like him? (this is not a sandbox server so i assume its a lot harder on RP servers) Im hoping for maybe an addon that allows for this. Any help is appreciated.