I think Ozonekiller couldn't be more right, simply ask the GM about securing a permanent position, which may not happen because these ppl that are in the group are most likely long time raiding buddies, and you are seen as a filler! Core groups tend to have a certain social structure and/or are cliques so moving from the outside to the inside is very likely not going to happen if it hasn't already! People tend to raid w/ ppl they like and mesh well w/ the whole group!, and they may see you as not one of those ppl although your performing well!
Worst thing that can happen from asking if you can secure a permanent raid spot is being told no, and the event that it goes this way, like Ozonekiller said, you could suggest that you would like to start a 2nd group, or have the GM have someone start a group 2, the problem w/ this seems to be that the 2nd group seems to get poached to fill a missing spot in group 1!
Really, you need to bring it up to your GM on a one on one and ask!, worst case scenario you will be exactly in the same position you are now but at least you will know where you stand and what to expect!
Ignorance can be cured through education,but Stupidity is chosen ignorance!