That's weird, since its a monthly subscription for pro, and it doesn't specify how long you get pro for. 5 people for a single month ($10) would be kinda lame, for example.
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."
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…