I can't tell you how many times we experience this - so frustrating. Especially since most first-time users often don't click the link to establish their password within 72 hours.
What makes this process even more frustrating is the fact that when you delete a user who didn't take action within 72 hours, you'll often get this error when trying to add them again:
Sometimes you need to wait several hours before inviting them as a new user again.