Just tell them your not ready to play "competitively/seriously".

As in, your not playing to win, you just want to play for fun. Extra casual, if you will.

In your friends' defense, league is a complicated game, there's 100+ champs to learn the moves and combos of, even if just to know to watch out for them, let alone play them. Then, you have to learn build paths for each champion you want to play alot of.

If they're playing to win, having you start with game knowledge and then learning to play, is faster toward their goals of those sweet sweet squad wins, even if learning to play the old fashioned way, is the most fun in your opinion.

You both (you and your mates) need to accept you want different things from the game, an then go from there.

Even if that means letting you loose into solo unranked for awhile lol.

Stay frosty, an GG's mate.