I tested this again as there have been a lot of changes to the sign-up dialog in the past months. The error message shown when the invite code is missing is still the same.
I think an error message, “Please enter invite code,” below the invite code field, similar to the ones shown below the email, username, password, and custom user fields, would be helpful.
Thanks @Moin, you’re right that this error could be improved. I’ve opened a request with our team to fix this up. At this point, I don’t have an estimate for when this work might be complete based on the team’s other priorities.