In order to join the Teachers group, you need to demonstrate that you're already fulfilling the conditions of the relevant paragraph in the first post of this thread, including "They contribute to the teaching resources of the forum by posting mp3s or videos, arrangements or compositions, or analyses or online lessons, and regularly provide help to the amateur guitarists of the forum." The best thing is to wait a while until you've posted a few contributions of that sort and then apply again with links to the relevant posts as evidence. When the time comes, you'll also need to request a change of username to your full name.
Classical Guitar Forum.
"Experience is something you don't get until just after you need it." - Steven Wright