Don't worry about it...
So any XBLA games with online achievements that are on your tag (apart from GA and SoR2) can go up on the list? You still have all those on your XBox?
Also, as far as an introduction goes - just go in to the main thread 'Viva La Achievers' HERE and say hi, and maybe a little about yourself... Everyone's friendly there, but some people have noticed that a new member has appeared and find it strange that they haven't seen anything of you!
So yeah, just make an appearance to say hello really, meet some other members of the group and maybe arrange some games with them!