The clue is on the page... "Feel free to hack your way in :)" Should take you a moment or two if that's your mind set, a little longer if you need to brush up on your JavaScript, or a little quicker if you Google a walkthrough :-)
If you just want to see what it's about then google works I guess, but if you can't get registered on your own I'm not sure that Hackthebox is the best place to start for pentesting.
I tried to do a SQL injection on the invite code form and they blocked my IP for 15 min. I guess some technique's aren't allowed.
EDIT: Switched my target to the front-end and was able to get in pretty easily.