We have a business account and are getting error messages when trying to add two users who have free accounts. This is an urgent issue we are facing in order to collaborate with a client.
SOLUTION:
Did not get a response from support yet, but I figured it out, and I'm hoping this will help those of you who face this problem in the future.
Even though you get the error message above, press "Save" again in the area where you add the user's name and email and see the yellow exclamation (my "save" is grayed out since I took this screenshot after the fact):
Good luck!
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