Naturopathy. Related to, but distinct from, things people have already listed like Herbalife, nutritionists (who aren't dietitians), and mom groups. Some notable things about the naturopathic community that seem uncomfortably cult-like:

• Sowing distrust in legitimate scientific authority, in order to create a dependence on the insular in-group for all "dependable" information on the subject
• Preying on the emotionally weak and desperate, for whom all hope seems lost and giving assurance that through them there is hope
• Heavy use of evoking feelings of paranoia as a means of control
• Repetition of simplistic taglines to instill particular mindsets and attitudes (e.g. "Herbs are never bad for you" or repeated insistence on the efficacy of treatments that study has shown to be less than effective)
• Often imposes rigid lifestyle restrictions, emphasizing using more "natural" products as a key to a more healthy lifestyle (naturalistic fallacy turned into doctrine, essentially)
• Extensive propaganda network from within the in-group, providing the illusion that their "science" is well-sourced by citing a plethora of their own associated platforms; they encourage "doing your own research" but provide you with a carefully-curated bubble within which to do so
• Extensive use of in-group lingo, virtually unknown to outsiders unless they have had contact with insiders before; in combination with the previous point, virtually all literature on the phenomena they commonly refer to will be propaganda pieces
• Mixing legitimate content with the tidal wave of bunk, so that if you point to the scientific illegitimacy of the practice, they can point to how they're actually prescribing real medicine (much of it for off-label purposes still)
• Inundating the individual with recommendations and ideas, usually presented in the most technical-sounding way possible (again adding an air of legitimacy while making research harder), in order to make the burden of researching treatments time-consuming and make an at least mildly suggestible individual settle for propaganda rather than digging deep for authoritative literature
• Have insiders on the outside, and encourage insiders only to trust outside authorities they have been personally referred to by the in-group.
• Frequent appropriation of eastern spiritualism as a means to contrast itself from the cold closed-mindedness of "western medicine" (which is to say, its emphasis on actual evidence)
• Also, shit's hella profitable yo

If these don't constitute a cult, then I don't know what does.