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.