1. What is the most popular class for people to get into racing.
2. What are preferred machines to get started.
The most popular class are 600cc middleweights. IMHO the preferred machine for someones first 1 - 5 seasons is a lightweight bike.
Over the recent decade manufacturers have started bringing ultra-lightweights into the US; 250s 350s, etc. and those are also good to learn on. However, there are so few classes in which you can run them that it is hard to get enough track time to learn.
I'd say go with an SV650 or similar machine. Something in the 70hp range, ideally a twin, will enable a new rider to put their bike in more races than they probably have money and will enable you to learn.