Update: the NDA is now lifted, as the early access servers have been opened to PC/Mac players.
As of today, April 17th, 2017, ZeniMax has officially begun the closed beta for The Elder Scrolls Online: Morrowind. If you were invited, you should have received an email from ZeniMax and an invitation to a private forum to report bugs encountered in the upcoming expansion.
Please note, however, that by accepting the invitation, you are agreeing not to discuss anything about your experience playing the expansion until the NDA (non-disclosure agreement) is lifted. This extends to adding information to the wiki; any closed beta content that is found on the wiki will be removed.
You can access the PTS (Public Test Server) on which the expansion is set by clicking on the Settings icon on the game's launcher, then checking the box, "Show Public Test Environment." Then click "Apply" and select the option "The Elder Scrolls Online (PTS)" and press "Install" on the bottom right. The download will most likely take several hours.
For more information, see here: