Riot added a while ago that any rank can play together as long as it is in a five stack, and they'll try their hardest to try and even it out as much as possible. I recall seeing quite a few issues regarding this towards the start of the patch release but I don't see it as much anymore.

Anyways, was looking through a match history of someone that I thought was cheating just now just to see this

For more reference, this guy is 80%+ wr, 200 ADR overall, only season played and is around 500 - 600RR.

Whilst being able to play with your lower ranked friends is cool and all - but I think riot forgets the skill gap even between high immortal and low immortal. This is blatant abuse of the queue system to climb the ranked ladder.

Update:

After 13 more games of abusing this, he's hit radiant against plats. Very well deserved and hard earned.