Contact the affected users and explain why you cannot yet add them to your Webex organization.
(There's an explanation of the problem at the end of this procedure.)
Invite users to choose between:
If users want to use the same email address with your organization, they must delete their free Webex accounts. This is explained in https://help.webex.com/yjrmmp/.
Add the users to your organization in Control Hub, using the new or the existing email addresses provided by the users. See https://help.webex.com/v71ztb/.
Why did I see an error about the domain not being converted?
Webex uses email addresses as primary identities, so each email address can only be for one user account. Typically you would claim your email domain, then add all your users' email addresses. If some of your users were already in Webex using that email domain, you could claim them for your organization and optionally bring their existing Webex data along.
In some cases, your users have private email accounts, with email domains that you do not own and cannot claim. For example, free email accounts from third-party service providers, like Google or Yahoo. You can still manually add those users to your organization, unless they are already using Webex with that email address.
You can ask the users for different email addresses to add to your organization; the users can still access their free accounts on the original email address. Webex considers these as two distinct users: data and functionality are not common to both accounts.
You can ask the users to delete their existing (free) Webex accounts; this option deletes all a user's Webex data and configuration. However, the user can use the same email address with Webex, and becomes a new account in your licensed organization (rather than the general, free Webex organization).