According to a new post on the TERA EU forums, the game is nearing its alpha phase of development. Frogster EU Community Manager "Raven" indicated that testing will initially begin internally and quite small.
During the first alpha stage, we’ll invite a small amount of community members, which will beaded to our internal testers. This new alpha stage will be completely under NDA (Non Disclosure Agreement)."
After the initial testing, more and more players will be added to the pool over time.
Read the full post on the TERA EU forums.