Can't add users for project and failed to register also

Can't add users for project and failed to register also

I want to add some members in my project, but some were added successfully and others failed.
Then I try to let them register new account but the company name used when I created my own account, they can't use it again, is this a bug?

The purpose is we need to add members in same company to a project. Anything I could do for this?