I'm confused about what terminology to use when talking with people about these types of seminars.

The term "Large Group Awareness Training" sounds rather positive. The term "cult" may be too extreme for some of these, and could undermine credibility when trying to educate people about the dangers of these types of "trainings".

Any ideas about a good word to use that conveys the dangers without sounding like one is "overreacting"?

I find when I use the term 'psychtherapy cults', it gets people's attention. They always seem curious to know if an organization they are familiar with falls under that category. If you're not comfortable with that term, 'mass marathon training' might get your foot in the door.