Unable to add additional users

[bruceanderson]

We are currently on the Enterprise trial.  Initially I had created three users.  I need now to add two more, (one being the decision maker to purchase the license), however receive the below error.  I believe we need our domain whitelisted, as per other posts on the community. How can I have that done?

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You cannot add that email address to your account. Please see this Knowledge Base Article for more information.