Muhaba
فصبرٌ جم
I you see some wrong happening it's not only necessary that you say something but it's an Islamic obligation. I read in a hadith something to the effect that if you keep your mouth closed when you should say something then you're a sinner. Don't remember the exact words.
So if you doubt something is going on that might either mislead the people or put them in danger - as cults are known to do - then it's important to voice your thoughts. You can talk about it in a wise manner, even in a questioning manner so that it doesn't seem you're preaching. Like, you might ask the sisters why they aren't attending the Islamic class, is something keeping them busy, etc. If they don't talk about the landmark forum, you can bring the subject to it yourself, like say, so have you heard about this landmark forum, etc. and then turn the topic to the dangers and using your research as supporting evidence.
Finally, you can suggest that the sister do istikhara prayer to get guidance regarding this issue.
So if you doubt something is going on that might either mislead the people or put them in danger - as cults are known to do - then it's important to voice your thoughts. You can talk about it in a wise manner, even in a questioning manner so that it doesn't seem you're preaching. Like, you might ask the sisters why they aren't attending the Islamic class, is something keeping them busy, etc. If they don't talk about the landmark forum, you can bring the subject to it yourself, like say, so have you heard about this landmark forum, etc. and then turn the topic to the dangers and using your research as supporting evidence.
Finally, you can suggest that the sister do istikhara prayer to get guidance regarding this issue.