I think if you have to post something in a private group in order for it not to be found. Otherwise, as far as I've noticed, if you post in an open group then it will go to the community; but be categorized under the group for easy findings. Good question, however. I was just thinking about this....
When you create something, such as a discussion, you will see a window like the screen shot below where you can limit who sees what.
In a Place, allows you to choose certain areas within the site. For example, the discussion or document might be focused on Registrar information, you would choose Registrar. People who are then following the registrar's space are seeing that information. It is searchable as well, in case someone is not following the assigned space. In the example below, it is the Alpha Tester group. This is a secret group and only the people in that group would be able to see that discussion.
Specific People allows you to limit the direct interactions to a certain group of people. I would use this if the group is less than 20 (though to be honest, that might be pushing it). Only the people you have chosen can then see the discussion or document. This feature is great if you are working together with a couple of people on a project.
The MSOE Community means you don't care who sees it. You are looking for all kinds of feedback and discussion. Basically, a free for all!
So, that is a quick explanation of who can see what when.