User blog comment:TheTobarMethod/Mindset Theory: Solo vs Duo Que/@comment-7457593-20130408202838

Don't know about ranked, but in normals every time you queue in a group you alter the matchmaking and get harder opponents. The system assumes that when you're in a group you have better teamwork, then it raises your group's hidden elo. Probably in ranked it's the same.

Also there is the case when you queue with a low level friend, it creates a mess with mixed low lvls and lvl 30's that usually ends on those 30's smashing the lowbies.