Well... The fact that it became known does not speak much for their secrecy, and secrecy in this regard is a very relative term, even if the group ever intended it to be a "secret society Illuminate". Sometimes (and I've seen it happen all too often) someone accuses people of discussing things "in secret" only because they weren't a member and the membership signup was not obvious to a 3 year old. Without knowing more about the specific list and group, it is impossible to judge their motives based on an unsubstantiated claim of a "secret mailing list".
I've been a member of "closed" mailing lists before and continue to be to this day. It's generally a question of someone vouching for you. Example... In the dark early days of the Internet and the Robert Morris Worm incident, we had two parallel security lists. To get on the Zardoz list, you merely had to sign up. To get on the ISIS list, you had to have some vouch for you in the "bang path" (uucp notation) between you and them.
More recently, certain mailing lists, such as the recently defunct VendorSec mailing list,. required a discussion amongst the members for you to join. Especially, in security circles, there's a matter of trust and reputation and the very real problems of disruptors , some of whom are "state sponsored" (the government really doesn't like it when you can protect your privacy and your security - you should depend on them for that, right? They long for their good old days of ITAR). Sometimes (SERIOUSLY) some of those lists are there discussing things of a serious enough nature that we don't want the "bad guys" to have a heads up. Some of us have to collaborate in a trusted manner somehow and, yes, we're going to get accused of "operating in secret". But it's just a matter of knowing who you are communicating with and can trust them. This doesn't sound like that kinda list but I would love to know what list it was. There are probably a dozen or more lists on the net right now discussing this very issue, probably including one or more IETF lists. It's generally not a "cabal" and I've never found it hard to join one if you have the reputation to be trusted.