Like what's been mentioned. Sometimes your ELO makes life hard for your friends, but I wouldn't play without them. It makes life hard for them at times but XD tough cookies. I have found that if your friends are average and you make constantly give advice. Like "Kassadin just killed you twice, you might want to rush an early Negatron cloak" helps tons.
Sometimes your friends just haven't found their "Pool" of champions. Once they find the ones they really like they tend to get better a lot faster.
Luckily for me, I don't think my ELO is all that far off from them, but I tend to have the most consistent stats (Except this week I was playing like a proper turd)