Chris is our group leader, a sandy-haired man sporting a baseball cap and a caring smile. He wants us to talk about the first time we saw pornography. People in the circle shift uncomfortably in their seats.

Nobody wants to catch Chris’s eye. Slowly, people begin to speak. Somebody tells of finding the builders’ stash of magazines hidden in a skip, another talks of a boys-only school trip to Holland and discovering adult channels on Dutch TV. An older man, Rick, makes the point that, for his generation, a first sexual experience often came before widespread exposure to porn.

The cabaret star Christopher Green’s Prurience, originally developed with the Sick! festival, describes itself as “an experiential entertainment about porn”. But that’s not all it is. Green is testing the limits of theatre, the role of audience participation, the line between the real and fictional worlds, and just how much he can muck around with the audience’s heads. He has form in this department, most notably with The Frozen Scream, a disappointing immersive-theatre collaboration with the novelist Sarah Waters at Wales Millennium Centre in 2015.

Unlike that project, this is no horror show. Quite the contrary – it’s often very funny and genuinely discomforting, largely because the dramaturgy is much sharper and tighter. It immediately solves the crucial central problem of so much immersive theatre in justifying our presence in the piece. Punchdrunk deal with it by putting the audience in masks so we become shadows at the spectacle. In Prurience, we are all participants in an Alcoholics Anonymous-style therapy group for porn addicts founded by former porn actor Amelia Atkins (played brilliantly on film by Pippa Winslow), whose stick-on face of permanent concern makes her look as if she is suffering from a really bad case of constipation.

You leave your bag and your inhibitions at the door, stick on a name badge, help yourself to herbal tea and biscuits (“for those who haven’t yet dealt with their sugar addiction”) and allow yourself to sink into the welcoming world of self-help groups. We are invited to join in the group singalong and shut our eyes and raise our hands if we have ever run along a street for the sheer joy of it, or sat on the edge of our bed and cried. What’s fascinating is how prepared people are to play along, even though the entire thing has a lightly satirical edge. We are drawn in almost without realising it.

Soon we move on to the sharing: there’s a young biochemist who believes that his erectile dysfunction is a result of watching too much porn from the age of 11 which has shaped his neural pathways. And there’s a woman who gets off from watching gay male porn, although she draws the line at fisting because it reminds her too much of her job as a vet.

But gradually it becomes apparent from the fixed smile of Chris — played by Green himself — that there are rumblings of discontent among regular attenders at the group. What is its function? Is porn addiction a real thing? Are therapy groups such as Atkins’ Prurience merely cult-like money spinners, part of a capitalism conspiracy that on one hand sells us porn and then tries to sell us the apparent cure? Maybe porn, food, drugs and social media have simply become a substitute for the other certainties – such as faith – that we used to believe in.

Prurience shares similar territory with David Hare’s 2000 Royal Court play My Zinc Bed in the way its raises the possibility that therapy groups substitute one kind of addiction for the addiction of public confession and how it inspects whether anyone really wants to be totally cured of desire. But it is way more interesting than Hare’s play because of the way it genuinely tries to marry form and content. We are sitting in a room pretending we are in a therapy group that is discussing people’s disconnection from the real world because of porn. It’s a piece of fiction about an addiction that some scientists dispute even exists.

If you want an idea of just how discombobulating the entire experience is, suffice to say that the post-show discussion doesn’t take place where you might expect. There were a few dislocating moments when I genuinely considered the possibility that everyone in the room was an actor apart from the five people I knew.

Prurience doesn’t entirely succeed in keeping all the balls in the air, but at its fiendish, exhilarating best it makes you question not just attitudes to porn but how we experience reality.

• At the Royal Festival Hall, London, until 30 July. Box office: 0203 879 9555.