Not sure if this helps. Create your space/room add the EURL bot, send the EURL bot link to your mailer and ask folks to join. I saw you can do this by adding a dist list https://help.webex.com/en-us/article/dmfxm3/Add-Contacts-to-the-Company-Address-Book-On-Your-Webex-Site - but you need to be admin. If neither suit you needs here, suggest to open an internal support ticket with the Webex team/eng to help.
Hope this helps.
Please mark this as helpful or solution accepted to help others
Connect with me https://bigevilbeard.github.io