After a match ends, you can select Party Up in the post-game menu. Selecting Party Up will let your teammates know you want to form a party and continue playing together. If a teammate chooses Party Up before you, selecting this option will add you to that player’s party.
The green icon only indicates players on your team that are trying to party up. It will also appear next to the Party Up button when anyone on your Team is trying to Party Up. If an entire team uses Party Up their blinking icons will change and display that they are in a party.