We just rolled out new functionality that unifies the experience of inviting a new user to your WordPress.com blog. We haven't announced this yet though (still working on the post). The form includes three fields: a WordPress.com username or email address, the role, and an optional personal message. If you enter a WordPress.com username, it will look up and use their email address. If you enter an email address, the invitation will be sent to the email address.
@airodyssey The user 'mmedejantee' should be able to use existing WordPress.com usernames in the new form. Can you convey that information, and then ask if it works properly for them? I'm happy to take further feedback on this thread.
Code Wrangler, Automattic