They don't have any special priviledges to enter the Otherworld, and most of them don't know it exists. In the first game Alessa grabbed a bunch of victims into her world to punish them (same with the movies), and in the third game they get pulled in because they're already part of it or have reasons to be there like in the first game. There's the possibility Claudia uses her perception powers to make Heather perceive, and thus bring in, ordinary cultists for her plans, but that's about it.
Thanks for the response that makes sense now. I know the town has always been a place of power but when did people start experiencing the other world and the fog world? Was it the power of Alessa that tapped into Silent Hill or was it always such?