Well, there are tons of open casting calls in NYC for different things. However, many of the dance ones involve industrial shows (dancing next to a car at the auto show etc.), which are fine but not anyone's dream I'm sure; or small modern companies that don't pay anything; hip hop dancers for an unknowns video, etc. BUT, when you don't have to pay rent, you can go take as many classes as you can and that's often where you get noticed - by someone who has a company, or is choreographing a show, etc.
As far as the union, often there are Equity only calls. Anyone can go, but the Equity members are seen first. If you are lucky and get to dance, AND you make it, you then can join Equity b/c you've been cast. Also, some Equity shows have open calls - like touring shows of Barney, Sesame Street Live, etc. So everyone gets seen there and you join the union once you begin with the show.
[/lesson on NY dance scene]
