The annual mad rush to destroy the FIFA 17 WebApp servers can begin, as the service is technically available now. It’ll be getting slammed, so if you experience problems logging in, that’ll be the reason why.
If you get a message saying you’re “not invited” to participate early (even if you’ve played a bunch of previous FIFA titles with the same account), that’s a bug. EA are working to resolve it.
Assuming you’ve got a prior account, and assuming you’re able to get in, the WebApp will allow you to make an early start on your Ultimate Team stuff for FIFA 17 – even if you don’t own the game. If you’re unable to get in, then while you wait you might want to browse the FIFA 17 FUT player database (may currently be incomplete), or read an article EA have put up about how Chemistry will work in this edition.
FIFA 17 will be released on 27 September in the US, and 29 September in Europe.