The developers behind Rocket League, Psyonix, have stated on Twitter that they are actively looking to create cross-platform party support. The feature will be coming at a undisclosed time sometime this year, which is great for Nintendo Switch owners and Rocket League fans in general.
Hey there! We’re actively working on cross-platform party support for a 2018 release. :)— Rocket League Help (@RL_Support) January 21, 2018