it's an order. but as with most old orders, there are plenty of spin-offs, all claiming traditional lineage...

they're open to anyone. if you're interested, mail or call them, or walk into one of their offices. most large towns in europe have a meeting place, which is open to public.

in the netherlands, they still have (or had) their own schools for ages 6-12 ("Ryckeborgh"). their teaching methods were interesting (take a pinch of montessiore, a whole lot of steiner and a truckload of symbolic hermetism, and combine it with a lot of classical science).