They assume that everybody is working for a company and every company has it’s own e-mail domain. That’s why you can’t sign up with a gmail address. Maybe that would be a good topic for the idea forum.
Some people are freelancers wothout their own domain. And I also don’t see a reason why a privat person should not do something with mendix (Or am I missing something?).
Strange as it may seem, your company name is indeed not something you can change here by yourself. Send a change request to support.mendix.com, they will do it for you.