Hi Mark,
Maybe some stray cookies hanging around in your browser or Slack app. You should be able to join with this link: https://join.slack.com/t/mendixcommunity/shared_invite/zt-2fy9x4p8i-aB7n2R0mhrGtvjEo8ZJoNw and AFAIK, there is not restriction on the email domain you can use to sign up for the Mendix Slack. If that link doesn't work, feel free to contact me at MarkMendixSlack@brooktroutpartners.com and I'll try to get you signed up for the Mendix Slack community.
Hope that helps,
Mike