Consisted of the exact same portions of players (meaning if one team has a duo, the other must as well, and if the rest are solo players, the rest must be for the other team too, or a group of three and group of two with a solo etc.) and in similar SR (the usual 500/1000SR restrictions), and that’s it.
The matchmaker should not stack odds against grouped teammates, it should instead require the opposing team to also have the same coordination, every time.