Yup, this is what's happening. If the mission isn't set by the team leader it will default to the level of the first person who enters if they also have the mission. The only thing you can do to minimize the problem is for the lead to get and set the mission before putting the team together. This way if anyone enters early it will be the at the lead's level, not the level of the person who enters. The downside is that this allows people to enter before everyone is ready and unfortunately there isn't anything you can do to stop people from talking to Posi and starting a Nemesis run or skipping the ceremony (I posted in the suggestions forum that the event should be altered so that only the mission owner can talk to Posi, which would eliminate this problem).
Griefing has been RAMPANT for this event, and yeah it's been infuriating. I don't know why people gotta be assholes. But here we are. Unfortunately there isn't a lot that can be done about the level issue. It's just how the game is set up. It's not something limited to the Spring Fling.