The Azure Enterprise Agreement portal allows large enterprise customers of Azure to manage Azure subscriptions and associated licensing information from an easy (well.. seemingly easy) to use portal. On-premises and cloud license details are able to managed under a single EA which make it streamlined for those who manage that sort of thing.
The details in this blog post are not longer relevant. I've removed the content to avoid confusion.
Follow or start a discussion for this blog (Azure Enterprise Agreement (EA) portal and adding an existing Office 365 tenant) on Twitter. If you're after something more in depth, or want to ask me an expanded question: raise an issue in my open GitHub AMA repo.