If a friend joins a tournament that hasn't started yet, such as "Bullet Shield Tournament" that starts in 14 hours, a notification shows up on the top left of the homepage (desktop view) that states "[PlayerName] competes in [Tournament Name].
I suspect this only happens with tournaments that can be joined hours in advance, but it might also happen with shorter term tournaments.
I suspect this only happens with tournaments that can be joined hours in advance, but it might also happen with shorter term tournaments.