This misleading error can occur if the person you are trying to invite someone who does NOT have a gmail address. You have to have a Google account to be a member of a Google Nest household.
@MplsCustomer, appreciate the help.
@LexiPaulson, would like to know if the person you are adding is using just a regular email or a workspace account? Also, have you tried linking another regular email account to see if that would work? Just for us to isolate if it is on your friends account that we are encountering it.
Keep me posted.