This just happened to me last night and the suggestions I've received so far haven't been helpful (except possibly one). I deleted the FaceTime app and it continued. I individually blocked each number and it still continued. FYI, It appears to be a form of a prank (as opposed to fraud) where people are fishing for irate and/or confused reactions.
I chatted with Apple and the person/bot hadn't heard of this happening, even though Googling "FaceTime Group Spam" results in plenty of reports, including articles and Apple discussion groups dating to 2020.
Apple Support suggested setting FT to receive only calls from people I know, but that means I risk missing FT calls from people I don't know who legitimately need to reach me. Another suggestion was the ol' hard restart, which of course did nothing. I then went through two more levels of Apple Support and they only suggested the same things I'd already tried.
Next step is to contact my phone carrier to report all the numbers I took screenshots of from FT.
BEST TRY SO FAR: I found an online suggestion to unlink my phone number from my FT so that people wishing to FT me can only do it through my Apple ID and not my number. That may have worked, as I haven't gotten more calls yet, but it's not ideal since a lot of people who FT me use my number.
Good luck! I know how ridiculously frustrating it is and nobody wants to have to change their phone number.