Its pretty easy I think, someone just starts a group and says "Hey everyone, join in my group!" We can set up RP times here on the forum as well as keep track of stuff.
I played a little last night on SL, had a blast and made some friends pretty quick in Bronyville. Though I think some of the series characters are taken for official RP in Bronyville and Canterlot. I became friends with the official Rarity and Fluttershy last night, both great ppl! It was pretty populated and the streaming fan made pony music isn't bad ether.
I was able to go shoot Zombie Commies at the Canterlot Arcade, do some light shopping, and watch Friendship is magic cartoons at the movie theater. I even physically got caught in a cage when I was in the Everfree Forest.
I made myself just like my avatar here, except with wings. I can help anypony get started, but most everyone there is helpful.
So, who wants to get this thing started? It'll take a real leader with hooves of steel and a heart as golden as a summer apple!
Update: I just became the official Derpy of Bronytown (SL's ponyville) the staff are really nice and helpful as well. Stop by and rp with us!