ive never met a satanist that wasnt an atheist. there is a big difference between a satanist and a devil/satan worshipper. my understanding is in the old hebrew a satan is one who is in opposition. even the old testament gad is described as a satan once. so satanist have litterally named themselves as the opposition (to christianity and the other abrahamic faiths)

It sounds as though the name is used in a tongue in cheek way, so they could possibly be a group of atheist/agnostic/free thinkers who would be refreshing to join. I have not had any personal contact or experience of them.

Way back in the 80s, I had a GF who was a LeVayan Satanist. She lived in a sort of commune - a huge house in Birmingham - with 9-10 others. Apart from one guy who was a complete dork, the others were about the nicest, kindest and most delightfully insane people you could hope to meet.
They don't believe in gods or devils as such and put no restrictions on each other. I considered joining but never did.