Invites will be going out to select players across North America and Europe, although the NDA forbids participants from even mentioning that they're part of the program. Those who have pre-ordered the digital expansion will have "preferred access" to the beta, so that seems like your best bet if you're interested.
Rise of Isengard will release on September 27th, and contains a level cap increase to 75, three new zones and a 24-person raid against the dragon Draigoch. The expansion heads deep into the territory mentioned in J.R.R. Tolkien's The Two Towers.