Well, you could try filling up the form in the front page... xD. Just fill it up and you're in! We need more members, and music oriented guys are a rarity nowadays, so all the better! So you're probably in, but fill up the form, just to be sure.
As related to your question : Nobody here is an expert in music. So we will obviously make new music given the inserter and custom music.
The Plotline isn't decided yet, which is why JNathan is making one. If you feel you could help, great.
JNathan, I know your storyline, but Shadow Pokémon is a subject already implemented in Pokémon XD - Gale of Darkness and Colosseum. Try racking your brain harder, make up something else if you agree with me.