On the Get Satisfaction platform you are able to have both public and private communities. They both have the same set of platform features and the same customization elements.
When a community is public, people are able to view the conversations within the community without being a member. However, they will have to register in order to participate. Search engines can crawl and index the community conversations.
With private communities, people are unable to view the community content until they have signed in. Search engines are also unable to crawl private communities. You can read more about private communities here.