You probably don't want to force them into it.
Case and point: Last game I was forced into ADC, I asked to not get it, I offered to switch because I am very comfortable playing a tanky CC support like Blitzcrank Alistar or Nautilus, I am very comfortable in the jungle and both solo lanes no matter what I need to pick. I am not comfortable as an ADC, the role that carries the most weight in the late-game teamfights yet can die in a heartbeat, especially since the enemy could easily pick a dive comp. They did indeed pick a dive comp to annihilate me and our Lux and a pokey/sustain lane bot to zone away me and Blitzcrank to ensure the enemy Jinx got a CS advantage
Guess who received all the blame when Malphite ulted the entire team and Talon dove me straight away? Guess who received the blame when our CC'less top laner couldn't stop a teleporting Malphite while he himself never teleported to dragon fights, guess who received the blame for being zoned by the enemy because our own jungler was invaded by Shyvana with 0 help from top and mid lane? Guess who received the blame when our Lux just barely missed her critical light binding that would've saved both our lives?
Guess who was told to uninstall/commit suicide/never play ranked again/deserved to be bronzeV by their own team
Yes, you should have a GENERAL idea of how to play each role in ranked, but everyone has preferences and if you place a jungle main against an occasional jungler, or an ADC main vs a occasional ADC, then you're gonna run into problems