Because I'm so shy and it is difficult for me to open up to people, especially if there are a lot of people around, I've never joined local reading groups. But I have been a part of a few online reading groups. I haven't joined a general one yet, but 2 book clubs surrounding a couple of series of books and also a writing club offshoot of one of those book clubs. So, that is how I have found them. Fansites or links from author pages from the series that I enjoy. You can probably find ones based on comic series or various particular superheroes if you want as well. Good luck with it, reading groups always did sound fun to me. 
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