Not sure if this still happens, but for groups of 4 that used to want to play together, there was no way to lock the team or kick from lobby. So what would happen is you'd get match-maked in as the 5th player, and as soon as the game started, your team would kill you. This would happen about 25% of the time with random matchmaking.

I ended up quitting R6 Siege because of the toxicity and constant slurs on voice chat. It's a shame because it was otherwise my favourite competitive FPS.