There are occult elements in the HP series from the start, but many are used in naive ways, which tells me Rowling probably wasn't a full-fledged member of the death cult...yet.
Once you reach a certain level of fame, fortune, and influence, someone will inevitably approach you with an offer for your soul. It won't be obvious what you're bargaining for, at the time. (If you're not a Christian, you probably won't notice all the red flags.)
For JK Rowling, who was richer than the Queen herself, the death cult had no choice but to recruit her into their ranks. I believe something like this happened to JK shortly after she'd written her HP books.