I've given this some thought. I've been a dancer in this situation, and heard other organizers ask this question.
I don't like the idea of asking someone to leave. I do think this is antithetical to my values as a dance organizer.
I don't like the idea of gatekeeping and presuming I know how well a person may succeed not - especially since contra is a team sport.
I also want to raise the idea of economic injustice - many folks carpool, or, in NYC it may be a pain to get to a location - even by mass transit a new person could really need the help of a friend. Especially if it's a person from out of town; having lived in and around NYC for a decade, it can be _daunting_. So I can totally see some experienced dancers wanting to bring a friend, and having them come later is not an option. Forcing them to come later may be easier for folks with financial means, which is why I'm saying this may touch on an issue of economic justice and privilege.
An alternative I might have - and, totally with the benefit of hindsight and reflection:
I think if I'm in this situation in the future, I will do the following:
1. Caution the dancer of the expected skill level.
2. Offer them to sit and enjoy the music for free / donation instead of required entry fee.
3. Recommend that if they're set on trying it out, to do so, but if they're not getting it, come hang out and stay, with fee refunded. (Like a satisfaction guarantee.)
Ideally, if they do stay and sit, some dancers will need breaks and sit with them and make them feel welcome.
In dance,
Julian Blechner