UPDATE 12/01/21: Ubisoft has reached out to clarify that players can still matchmake "as usual" with other players, but to join a "pre-made squad", you need to be a squad with a pal to accept an invite.
Original story follows.
ORIGINAL STORY 10/01/21: Ubisoft has released a hotfix to help stop players using an exploit "to force themselves into squads with other players" in Rainbow Six Siege.
The change is in response to a recent spike in hackers forcing their way into private lobbies and refusing to leave, and there's currently no way to kick players from your squad. Right now, it's only been deployed on PC.
"To fix an exploit where players could join lobbies uninvited, we have released a hotfix that will require PC players to have at least one friend already in the squad," explained a tweet on the official Rainbow Six Siege Twitter account. "Thank you for all your patience!
"To clarify, now players can only join games in which they have a friend already in that squad (whether it be the squad host, or someone else already in that party)," the account clarified a short while later. "Players can no longer use an exploit to force themselves into squads with other players."
To clarify, now players can only join games in which they have a friend already in that squad (whether it be the squad host, or someone else already in that party).— Rainbow Six Siege (@Rainbow6Game) January 8, 2021
Players can no longer use an exploit to force themselves into squads with other players.
For most players who have a regular fireteam this is unlikely to be problematic, but it will temporarily prevent those who rely on matchmaking with random people from getting online.
Just in case you missed it, Rainbow Six Siege has a free next-gen console upgrade. It means existing players of Ubisoft's hugely popular competitive FPS can get the PlayStation 5 and Xbox Series X and S version at no extra cost when upgrading within the same family of consoles, which run at up to 120 frames per second and 4K resolution.
At the end of 2020, Microsoft added Rainbow Six Siege to Xbox Game Pass after teasing a cryptic tweet on its social media channels.