ask at the beginning of champ select if any one is auto filled... and that should start the converstation
but lets think outside the box just for a min i get it people dont like autofill... but over time as auto fill is here and makes people play roles they arent use to... yea you might lose a game or two at 1st but all it is doing in the end is making every one better players by exposing them to more of the game, roles, champs, match ups and so on...
and the more knowledge you(the community/player base) has about the game in itself makes the games better.
as for me i know i can play 4 roles... the one i never play is mid... but there a hand full of champs i can be ok with... so unless i wanna just play top my next game... i chose fill every time. and i have gotten all 5 roles by doing this.