Well, there's gear for one thing.
Of course any new player won't have the gear. Without gear, you're ad a major disadvantage.
Also, try incremental guilding. Join a certain guild you can get into, which occasionally does a few epics, then get a hang for it, and leave eventually. There's no reason not to accept a new guy into an epic run; but if he has bad gear/build, why would you even consider it?
Of course, there are many groups who won't take a new player into an epic, but if you're shot down every time, maybe it's not anything to do with them after all.
PS Guild renown has nothing to do with it; it's completely irrelevant.