How can I add students to an existing classroom? If I just try, the new usernames Duolingo produces are called 'None'. They are not able to log in with that name.
I've got the same problem! This is recent because I had no problem with this feature until recently. Please report it here https://support.duolingo.com/hc/en-us/requests/new if you haven't done so already. The more we report it, the higher the probability of getting the staff to work on it.
I've just tried it again today, and the new usernames are normal now... Except that they still don't work!
I ended up asking them to create their own accounts and to join the classroom by going to https://join.duolingo.com/ and asking them to enter the classroom code that you can find in 'classroom settings'. It worked!
Don't ask them to use the 'Progress Sharing' option presented in the setup instructions PDF because that also does not work.
I hope this helps.