I noticed that while playing a competitive match sometimes people leave but nobody seems to join, usually tilting the match in favour of whichever team has the most players. Are other people incapable of joining a match in progress or is the match making system simply broken at this point?
A problem with competitive play
It’s not broken, people just leave. It’s unranked people that don’t realize it’s stopwatch.
People can’t join a Competitive match in progress, you could say they are commited to the match. Those ragequitters were probably banned from the mode.
Better for us, right? 
I’ev been playing a few more competitive matches and I have to say the system does indeed work a little better than I had experienced in the first few games.
I started competitive game today. After 15 sec we were 4 vs 5. After 2 min of that game I get sick of it. Keep playing about 20 min 4v5 is annoying. Not fun at all. Now Im gonna be punished for that. For what? I dont wanna waste 20 min of my life !
If a player drops, i believe you normally get a msg after a few minutes, saying “XXXXX left the game, it is now safe to leave the game” So i take it that after this, you could just leave the server, and find a new match, without getting any punishment for doing so.
It does looks like most people keeps playing on, and i think the problem is. That the msg comes with small text, for a few seconds, so probaly many who never notice it, to caught up in the game.