Blunion
They have to join a league. They can't do a couple things without joining a league first. I have the same problem in my league.
Try not to boot them for a certain number of races at least. This might be harder to do if you're racing every day, but in my league if they aren't active for 3-4 races, that's a week they weren't active for.
In my daily leagues for Rookies its fairly easy, team signs up, does 4 races and doesnt sign in for 4 days, there obviously not coming back so I kick them :(
This weeds out the people who obviously only download the app and sign in to have a look and then move on.
But I think the question was more, do the rookie teams get to choose what league the join or are they randomly dropped into leagues? I know I got to choose my own league when I sign up recently via the website, but not sure about the app.