Strangely, If I go to the website and send the invite it works fine. From the software, not so fine .
I am having an issue with 4 or 5 email addresses, however the most pressing is
kim underscore ankenbauer at yahoo dot co dot nz
I tried to reproduce the problem, but was unable to do so. I had no problem to invite Kim from Family Tree Builder.
Can you send a screen shot of the error message you receive?