Aren't threesomes and foursomes just special cases of group sex, which is anything more than a couple? So you could just keep going and talk about five-way/fivesome, sixway, and so on but it is all "group sex".
Headbanging ape from cold North 🤘
A poem for your enjoyment. Little something that came to me a couple days ago
https://www.lushstories.com/stories/erotic-poems/the-mistake-4