Erotica and BDSM is not everyone's cup of tea. It does not make it wrong. You may want to find it flattering that of all the years she's known you, she has finally worked up the courage to invite you into being part of an aspect of sexuality she enjoys. On the other hand, she should have communicated with you before inviting you to an erotic group of that nature.

You may want to tell her, "I appreciate the offer and I respect your interest in this activity and I am grateful you trust me enough to be open with me about this, but this is not something I would like to be part of and would prefer not to discuss this topic on Facebook."