As stated in the Code of Conduct, teaming up in groups larger than the game mode allows is not permitted (for example, do not work with other players in solo matches).
You can report players in-game via the scoreboard; on the scoreboard, click on the exclamation mark next to the player's username to submit a report.
You can also submit a ticket to our Customer Support team for further investigation. Please make sure to include the usernames and a replay.
The replay should preferably show as much as possible of the match, clearly showing the players working together (e.g. two or more players defeats another player, and continues to move in the same direction afterwards). If you are killed by players that are teaming, please continue to spectate them after your death as well, so that we have as much proof as possible.