No the Cursillo Movement is not a secret society. Il has no secret knowledge, no secret ceremonies, no exclusivity. On the contrary! Its aim is to let the world know about the best of news (that God loves us) by the best of means (friendship). It is open to all Catholics (and other Christians as well) from all walks of life. What cursillo is all about and what goes on during the weekend may and should be told to anyone.
Why then is your friend so reluctant to talk about his experience on his return from a Cursillo weekend? It is probably due to a minor detail I spoke about In our “Frequently Asked Question” no.2 : “What goes on during the Week-end?”. After explaining how a profound spiritual experience is not easily communicated by words, I wrote: "there is a much more mundane reason for not wishing to divulge the entire contents of the weekend. There are a few surprises along the way – pleasant surprises, which bring joy and spontaneity to the experience." Indeed, as men attend the weekend separately and before the women, husbands are ask to avoid revealing those minor aspect of the weekend to their wives, so not to lose spontaneity to the experience for the future candidates. But some are focusing too much on these “surprises along the way”… and forget to testify about the fundamental of their experience.
Fortunately, by going through our “Cursillo” section, I think you will find answers to your questions and realize that our Movement is far from being a secret society.