For purposes of 'competitive' MM what ranks can queue along side unranked players when there is not a full 5 man party?
In a 5 man lobby you can have all ranks, in other cases only 5 ranks plus or minus.
If you have unranked players and the lobby is not full, the ranks you can play with are from Silver 1 up to Master Guardian 1. It's made to disable CS:GO boosting companies and smurfs to queue with low ranks while playing on high rank. For example unranked account can't queue with Supreme, in the past it was possible so you could play as Supreme against MG ranks because your friend was on Silver or unranked account with very low hidden ELO ;) It's not possible anymore.