Please fix this, or at least point to the readily accessible solution of switching to the old admin experience to sent the user an invite. 

<rant>

This answer should be able to be found in the documentation and forum posts.  Instead, one wades through post after post that has the response "I made you a ticket" or "it looks like your problem is solved" instead of a useful response that points to the solution.  Perhaps just delete or hide these old posts that contain no helpful information and simply serve to obfuscate a solution that is within reach?

I dare say, this must be an issue for virtually every single onboarding.  Someone will have used Box with an external collaborator, or tested Box themselves, before the company as a whole is onboarded.  The forums are full of people asking how to invite a user to an enterprise when they already have a personal Box account registered to their work address.  Please consider each one of these forum posts as a vote to bump up the issue in your internal bug tracker.

</rant>

Thank you for your time.