Do you mind if we let Henry explain this one?

HENRY: You know, we always called each other "goodfellas." Like you said to, uh, somebody, "You're gonna like this guy. He's all right. He's a good fella. He's one of us." You understand? We were goodfellas. Wiseguys.

So, according to Henry's narration, "goodfella" is a term of endearment, a way to mark someone as one of your own. Of course, the fact that these self-proclaimed goodfellas spend the film's 146 minutes hijacking planes, cheating on their wives, and whacking people left and right is more than a little bit ironic and really stretches the meaning of the word "good."