Neither the participants nor the experimenters know who received which treatment - so all the participants are told they are attending a Landmark Forum, the experimenters have two addresses to hand out the participants, but they don't know which is the real Landmark Forum and which is the carefully-crafted applause lights / horoscope-style statements that apply to everyone / empty words / positive-thinking drivel lecture.

Probably one of the experimenters would have to find a third-party motivational speaker of some sort, work out the content-free speech, and then excuse themselves from the rest of the experiment.