Invites will be distributed among players who complete a survey on the site, although the developers have emphasised that numbers will be kept rather low during the initial months, with a gradual increase as the launch approaches. Later in the game’s development, large numbers will also be needed for AvA and stress tests.
In the announcement, the developers also revealed that their beta would differ from many other games. Instead of players receiving a beta pass that gives them access to the game for the duration of the test, Elder Scrolls Online will operate on a “beta event” system. Players who receive access to the beta will be invited to take part in short tests related to specific parts of the game such as PvP, crafting, or questing. Testers will also be required to give feedback on the test through surveys, bug reports or simply posting on the beta forum. Because the intent of the tests will vary from event to event, the developers have stated that getting into one event does not mean you will get into any of the other ones. Initially the events will run for short periods of time with only a few each month, but as the beta gears up the tests will get more frequent with longer play times.
For those interested in reading the official developer post, it can be found here.