When you first create a Hopin account, you'll be prompted to fill out your profile information. This account needs to be filled out to join an event.
If you can't finish this process, or run into any errors, close the browser, reopen the browser, and return to https://hopin.to.
Click "My Account."
And go to "Profile" on the lefthand menu.
You can complete your profile information here and can join an event thereafter.