You are right. What I am saying is that secrecy of an online group is totally dependent on human choice and decisions. When Facebook (or anyone else) claim that they have secret group ... they are not technically secret because their secrecy is dependent upon non-technical aspect, i.e, humans.
BTW, the analogy of bank website isn't equally applicable because people don't hang out at their bank websites most of the time.
There is actually no technical way possible for having a truly secret group.