Just joined Insider

Do I need to wait for an email to invite me to play on PC, or do i have to wait until the next flight?

You’ll need to wait until the next flight I’m afraid—participants for this flight have already been selected and invited.
When the next flight starts you’ll receive an invitation via your Insider email if you’re selected; the current flight has about 10,000 participants so chances are quite slim at the moment.
Just make sure that you’re green across the board here, including being opted in to ‘Communications Updates’—if you’re opted out of them you won’t receive emails, including flight invitations
If you’ve got everything sorted on your end then you should be good to go whenever the next flight happens (assuming you’re lucky enough to get picked, of course). If you want to keep up to date, the Insider forum and Postums’s Twitter are your best bets.