Party Skill and Matchmaking use a Weighted Average that blends the average skill of all players. The Weighted Average then skews that average to the highest-ranked player.
Grand Champion Level
This system rewards lower-Ranked friends to catch up in matches. Lower-Ranked players will earn more for beating higher-Ranked players and less for any losses.
Below Grand Champion
Players will always earn 1 Division Point if they win (unless they're on a win streak), or lose 1 Division Point if they lose, regardless of the skill of the opponents.
All Rocket League Sideswipe matches count towards your overall rank.
For more info, check out: What are Sideswipe Competitive Ranks?