The actress details the encounter in her new memoir, "Rebel Rising" ... explaining how she received a last-minute invitation in 2014 to a shindig at an unnamed tech billionaire's home on the outskirts of Los Angeles.

The supposed Royal -- who she describes as male and "fifteenth or twentieth in line to the British throne" -- was seeking to flesh out the rich guy's bash with more women ... which is why she got an invite. RW didn't name the Royal ... sounds like someone far-removed.

What Rebel didn't realize at the time ... the party, which was medieval-themed, would transform into a sex-fueled romp later in the evening. She writes ... "They weren't talking about a boy-girl ratio like it was a year-eight disco. They were talking about an ORGY!"

The "Pitch Perfect" actress says she attended the party in the hopes of finding a boyfriend, but didn't predict what unfolded next. She says ecstasy started to get passed around ... which one of the hosts told her was for people to start feeling ready to bang.

Rebel says she bailed on the party ... obviously, she didn't wanna lose her v-card in a setting like that -- can't really blame her either.

This is just the latest chapter from Rebel's memoir to spark chatter. Remember, she found herself beefing with Sacha Baron Cohen with the finger-in-the-bum allegation -- something he has vehemently denied.

Speaking of that chapter about Sacha ... it's been redacted in the U.K., something her publisher confirmed -- and it all has to do with laws across the pond. It's unclear what laws exactly were in play to force the publisher's hand ... but Sacha won that battle for sure.