I also received such an invitation from a prolific Medium author in response, allegedly, to a comment I made on one of her articles. Because my comment was slightly critical & it appeared she had taken my words in the spirit they were offered I wound up going further down the rabbit hole than I might otherwise have done. Pretty clever, or just lucky perhaps. But things went dodgy pretty quick. This whole "Medium Writers Union" is entirely a scam run by people who have nothing to do with Medium.

In fact, I see the account whence came the invitation now shows as "deleted user".