New people in raids are not a problem, without new people to eventually become old people we would slowly end up with no people. And that's not good!
I prefer, however, that the person lets me know as soon as they join the group, or even better through a tell before clicking the LFM. If possible (I.e. if there's room) I'll try and get somebody you can follow around to try and learn your 'role' depending on build/raid/etc...
If you find yourself having trouble getting accepted into others groups, you can start your own. It is preferred you don't, however, as you probably won't know what party makeups work best. If you do though, just PLEASE PLEASE PLEASE do not hide the fact that you are a first timer/relative newbie. Put it in the LFM and for Heaven's sake don't add Vets/TRs only or any of that nonsense when you yourself do not fit into that category.
The third option is to set up a 'Practice Run' LFM for anyone new to the raid. You basically go in with other people eager to learn it, the Wiki in another window, and try to run the raid, reforming after failures. These can be great fun if you let em. Oak and I did this when we couldn't find an Abbot group after slogging through the flagging process and had a blast. Occasionally someone more experienced will join to help you out and give you some advice, but the best thing for you is practice. The more you run a raid and see it fail, the more you learn how to prevent those particular causes.