@everything-bagel Partied Up players should be placed together if the team size is 4 or less (I think 5 also works since that option was added in a new update),
Also some things to note:
I personally have not been able to test this for myself because I didn’t have people to experiment around with, so I can’t really comment on exactly how it works. Unfortunately this seems to indicate that players will be placed into a max of teams of 4 when loading into the map, Although I did have an idea, what if you had many teams and put the actual 2 teams for red vs blue, in like team 50 and 51 and then used the other teams to place parties in the same team, eg A part of 4 joins the game and goes to team 1 then you move them all to team 50, and then another party of 2 joins as team 2 and you move those to team 51
Source: Party Matchmaking Update