In case you missed the email. Here's the email i got and here's where you can find your link Looks like you only need to invite 5 to get a one year PRO account so it should be pretty easy :D I don't know anyone other than online who does the 3D stuff sooo... if you've been wanting to make an account and haven't till now…
Says this at the bottom! One year is a pretty cool deal for me! "If you invite your friends to Sketchfab and 5 of them join, you get a PRO account for free, for one year. It's that simple. You can get a free PRO account only once. And only if you are not PRO already."