Les Soirees d'un Couple Voyeur AKA Evenings of a Voyeur Couple (1980) - Jean-Claude Roy

A couple get thrills of watching themselves have sex. Soon they realize that watching the other spouse with different people might be an even bigger turn on.

The film is also known as Les Enfilées.

Starring: Alban Ceray, Morgane Fournier, Brigitte Lahaie, Virginie Caillat, Brigitte Verbecq & Richard Allan.

Les demoiselles de pensionnat AKA Maidens of the Dormitory (1976) - Jean-Claude Roy

IMDB:
Several young women are abducted in Paris by dangerous gangsters who run a white slavery prostitution ring. Interpol sends their best female agent to infiltrate the ring as a potential buyer for the girls.

Cuisses ouvertes (1980) - Jean-Claude Roy

France Lomay's husband (Jacques Gateau) is a salesman of sexy undies who calls on Dominique Saint-Claire. She invites him to watch her and her husband (Dominique Aveline) have sex and then to join in.

On a later occasion he goes with her and her husband to a swingers' club along with Karine Gambier. The receptionist is a brunette, presumably Deborah. Another couple are already present (Sophie Duflot and Dominique Irissou).

Later Jacques Gateau visits Karine and the receptionist arrives to visit her and he watches a g/g session.

Lingeries Intimes (1981) - Jean-Claude Roy

A lucky guy inherits a lingerie shop and gets even luckier when his salesclerk and customers end up being nymphos. Catherine Ringer [Les Rita Mitsouko] plays a newly widowed bride who comes looking for mourning lingerie after her husband dies from too much lovin'.

Éducation anglaise (1983) - Jean-Claude Roy

It's 1930. The father of the young Sylvie kills his wife and the before himself committing suicide. The girl is entrusted to her uncle, a libertine with two women. He sends Sylvie to a boarding school, where the discipline is very strict. The director does not hesitate to resort to strict corporal punishment. The young boarders compensate for the rigours of their education while devoting themselves to sexual encounters. Sylvie is initiated by Claire, who becomes her best friend.

Nuits suédoises (1977) - Jean-Claude Roy

Synopsis:
Dawn Cumming and Siegrid Cellar transcend the screen in this amazing remake of the film "Rear Window" of Alfred Hitchcock. A couple watches the particularly perverse sexual games of their young and beautiful neighbor from their window. Excited at the highest point, the couple then seduce her and “train” her in a devilish trio. A very representative classic from the golden age of X French cinema applauded by a crowd of spectators when released on February 23, 1977 Alpha France movie theaters.

Comment les séduire AKA 1001 Ways to Love (1968) - Jean-Claude Roy

Synopsis:

A seasoned lady-killer introduces a series of Decameronic vignettes about men's resourcefulness in seducing women and the price some of them have to pay for it.

Maîtresse pour couple AKA Mistress for a Couple (1980) - Jean-Claude Roy

A man hires two assassins two kill his rich wife so he can start a new life with his mistress, but she easily seduces them and turns them against their employer.

Lingeries intimes [DVD version] (1981) - Jean-Claude Roy

Synopsis:
The young Marc inherits a lingerie shop. Sales have skyrocketed since he came along. The female clientele pours in. The fitting room becomes the scene of increasing intimate pleasure. Exhausted, Marc decides to hire Aline, a young, saucy saleswoman. This gem of perversity drew a record number of spectators when it debuted in Paris on October 21, 1981 in ALPHA-FRANCE theaters.

Brigade call-girls (1977) - Jean-Claude Roy

synopsis from imdb:

A mysterious maniac is killing Madame Chlo's (Danièle Vlaminck) call-girls. Leila did not return from her mission in Moscow, and Madame is sending some of her girls to the dentist before their missions all around the world. Jacques Lefin (Jean-Yves Gautier) is the detective in charge, but as he watches the girls, particularly the beautiful Gigi (Erika Cool), he is being followed, and risking his life"

Accouplements pour voyeurs (1980) - Jean-Claude Roy

Synopsis:
A call-girl named Gina (Christine Lodes) decides to open up a special type of "spa" with one of her clients Francois (Ceray). Francois recruits two comely ladies Sylvie (Lucie Doll) and Sabine (Morgane)...
The gimmick is that clients in action can see other clients while they themselves are being attended to. This causes a problem when a female client, Serena, sees her husband (Pontello) in the next room. The problem is resolved with an orgy.

Les Enfilées AKA Evenings of a Voyeur Couple (1980) - Jean-Claude Roy

How about a plot?
A couple get off on inviting people over and while one screws the guest, the other candidly films the romp.