If you are not with Landmark, then you're against it--automatically. In my case my ex-wife joined it. She tried to get me to join and a number of her friends. None of us took the bait.
The way she explained it made the Landmark Forum seem quite benign. The Forum is a three day marathon affair which takes place in room with no windows where the participants are under strict instructions only to do what they are told. This includes when you can use the bathroom, drink and eat. And since it runs from early morning to around midnight, there's a hefty dose of sleep deprivation also. Even at home the participants must do homework which cuts their rest periods to the minimum.
The Forum entails abuse, harassment, and humiliation of the participants to wear them down. France 3 TV made an undercover documentary of a Landmark Forum in Paris, called: Voyage to the Land of the New Gurus.
You can see the documentary here, where it's hosted on the Cult Awareness & Information Centre. The transcript of the program is here.
The program was made in 2004, so it's current. It shows how Landmark uses cruelty and bully tactics to get its way. It refused to discuss the program and indeed has persisted in trying to shut down websites that host it or link to it.
Once inducted members are persuaded to sign up new members, as I've said, and also take more advanced courses, at considerable cost. These courses cost hundreds of dollars.