My take on this topic, as a newbie, is that it's really important to send a tell to the group leader before you ask to join his group, when you don't know the quest/raid. (Especially true for quests after level 12/14, when doing something wrong can mean put the whole group into trouble). I always ask :"Hello is it ok for a first timer to join your group ?"
I have been kicked from PUGs many times for saying it was "my 1st time on this quest". It's not fun and it's a frustrating feeling. You wonder "what is wrong with this guy? why being rude ?". BTW, I never even join "byoh" or "know it dont die" kind of LFMs even if i know the quest. I figured i probably was better off not running with these rude Vet/TRs. It's a game after all.
There were exceptions to that and it's the Shroud, ToD and Hound raids. Leaders always want people that "know what they are doing" and even if i send a nice tell to the leader i have never been accepted in one of these raids this way. So i "sneaked" my way into these raids, not telling anyone i was a newbie. Playing a bard (people mostly expect you to sing and haste/rage and that's it) maybe helped me here. I tried to stick to the leader and did everything i could not to die. It went well and i've never been caught doing "something wrong". I just died once in my first ToD but i wasn't the only one and i was raised soon. These were also the most exiting DDO experiences for me ^^. First time ToD when you dont want to get caught dying or doing something wrong (but you dont want to pick either) guaranties a nice adrenalin rush !
My guild is quite small, with a lot of newbie and only a few veterans. When they're online, they come to the raids with me and send me tell to explain what is going on, but i'm lucky i have them, not all newbie know helpful experienced players i guess.
Something that surprised me is the cheer amount a newbies that obviously have no idea what they are doing, are not self sufficient at all and, even when the leader ask if there is someone new, will not tell it. These are the same people that run off and get killed really fast. So i understand experienced players want to be careful with who they will group with. (Off topic but i noticed half of them play monk, maybe i'm wrong but i've learned to be very careful when following a HOrc Monk that rushes ahead of everyone : these guys die and ragequite a lot !)
But please : if someone sends you a tell saying he's new but he'll be careful, please consider accepting him, even in your raids