@Sturgis,
Back at the office and can expound a bit (before I need to get back to the work that pays the bills). Basically a2k itself will become one community on a platform (called hurdd) of communities in roughly 6 months or so. It will be finishing the unfinished groups plan that is half-baked here. Like that plan there will be the ability to create closed communities (just like there are closed groups in the groups area) but able2know (and I suspect 90+% of communities created) will remain open.
Obviously if it is open to guests content can't be hidden from specific users when they log out but the goal isn't so much to create secret content so much as to prevent beating of dead horses and fighting ruining things for non-participants. One could always log out and still read it and that is fine, because if they reply (using another account, for e.g.) that can be acted on then and there's no problem with the post not being kept a secret, that feature isn't meant to support that.
There are a lot of different models of community, from their distribution models (one to one, one to many, many to many) to their relationship basis (people you know vs topics you are interested in) to their identity and reputation models (anonymous, pseudonymous, eponymous). While we will try to give communities the tools we can to make the experiences they desire we can't be all things to all people so will focus on being the best pseudonymous, many-to-many platform we can be.
As such there will be an emphasis on the open privacy model. For example, one of the things I was pondering on my run was that Facebook's model allowing you to set privacy on a per-post basis is far along the privacy/secrecy spectrum of models and is a powerful tool for that purpose, some of my ideas were to give similarly granular control to individuals. But that is one reason (of many) that they are not cited as much in public discussions as other much less popular mediums like Twitter where the account is either entirely private or not so you don't usually have to think about sharing a post and if you can etc. Even public posts on FB are carrying a lot of the baggage of their privacy model, which is based much more on close personal relationships than being a marketplace of ideas and much less well suited for the openness that makes for a better marketplace of ideas.
As such we'll not compromise the openness to try to optimize for this kind of thing, that is a fundamental, non-negotiable part of this platform's raison de'être.