Its held every two years, so you'd be aiming at 2009. The selection process will start during 2008, & the selectors will be looking at combinations with some form at BE intermediate level. It's very important that your horse is happy working in close proximity to others as the team dressage test involves the whole team (6 riders) performing the test as a ride in a 60x x20 arena.
If you want any further information PM me & I'll pass your details on so that some more detailed information can be sent to you.