Issue:
Whatfix shows you, "This domain is not supported for your enterprise" error message when you are trying to invite a user to your dashboard.
Probable Cause:
Whatfix invites can be sent only to those users who use the same domain. For example, if your email address is Kent@xyz.com, invites can be sent only to other users in the same domain. This is done to ensure that your authoring environment is secured to your domain.

You also need to be an Account manager to invite other users. For more information on who can do what see Whatfix User Roles.
Suggested Solution:
Add other domains (of partners, for example) by contacting support@whatfix.com.
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If the issue persists, contact support@whatfix.com