Green’s Dictionary of Slang

a devotee of group sex.

1970		Harper’s Mag. Feb. 54: Many columns of ads for wife-swappers, lesbians, homosexual, groupers, sadists, masochists.
1976		P. Theroux Family Arsenal 63: She was a groupie [...] they’d tie her up like a parcel. And then, you know, they’d do stuff to her.