In my experience, killing the level 7 boss 51 times so far, you just got to start it up and people will just join. Now that we are all 750+ you only need about 4 halfway decent people to get most of the way through it reliably, especially if you can figure out to chain supers. Last boss mechanic is usually hit or miss, but then you attract more randoms when they see you are close to the end. Good luck. 👍