Resources, advice, information and camaraderie for teens with mental health concerns of all kinds. The site mainly focuses on anxiety and depression and doesn't have many resources specific to PMDD. There's also a section for parents and other adults supporting teens with mental illness.
Hotlines (link coming soon)
The main US organization advocating for and educating about PMDD. They have lots of information about how PMDD works and how its treated. They're also a great place to start if you're in the US and want to get involved.