My uninformed impression is that these classes tend to use pretty light weights and lots of reps so that people can keep going for an entire class. So it's not going to be dangerous if you haven't had any experience with free weights, and lots of people in the class will be in the same position. And it might be a good opportunity to get comfortable with free weights, if the class format appeals to you.

OTOH, "light weights, endless reps" is really not the most effective from a strength-building perspective, so you may find that after a while you fancy moving on and using free weights on your own so that you can increase the weight.

gubernatrix wrote a post about getting started that way:

http://gubernatrix.co.uk/2007/11/how-i-got-started-with-free-weights/