This was split from the original discussion you posted to because this is not the same error those users were seeing.

Were you sending to a username or email address? Were you inviting multiple users?

Is there anything unusual about the user name? This is certainly not a general problem people are seeing and I can't reproduce it. You can of course also invite them with an email or directly from their profile page.

The most likely cause of this would be a mistake in entering the username (or about what their username is). Note that the name displayed on their profile would not be their username if they have added a different name to their profile.