Testing out my niche-how to ask people in a facebook group etc?

I often see that when people ask if they have a good niche, the reply is to see if they can find 10 people willing to pay for their coaching. A recent answer said to “go out and see if you can find your people gathered together in a Facebook group discussing their problems and eager to pay for coaching to help them feel better.”- My question is how do i ask them if they are interested in Coaching, if i’m not allowed to directly advertise my services in these groups? My niche is students premedical and medical students, and i’m finding facebook groups and instagram pages they’re in, but i cant directly post about my services, and I feel weird sending them private messages asking if theyd like to work with me. Any advice? Thanks!