As Temdee did point out, the cap is at 4500 MMR, so anything above 4500 MMR is treated as 4500 MMR. This means that anyone with 3500 MMR and above can queue with someone at 4500 MMR.
Lets just say you are a diamond in EU, and I'm unranked. We theoretically should be able to play together, because I'm unranked.
Problem is, my region is set to North america, and in north america I'm theoretically a silver. As of right now, the game is matchmaking based off the region your config is set to. For us to play together, I would also need to change my region to europe, so the game doesn't recognize that silver NA rank.
Now what I noticed in this is that I could theoretically be a copper in NA, switch that account's region to EU where I'm a plat, and queue for ranked with a plat in NA because the game is recognizing the plat in EU, not the copper in NA on the initial matchmaking query.
From my findings it would allow you to queue, but disband the party immediately thereafter with a matchmaking error. My guess would be that there's away the disbanding, because I've seen a lot of photos of diamonds with coppers lately.
The Unranked/Ranked queue situation has been something we've reported up to the team to look back into.