It is! You can set entry security in communities to be public (viewable by anybody), locked (viewable only by people who have joined the community) or admin-only (viewable only by the community admins), just like an entry in your own journal can be public/locked/private. There's no equivalent to posting security filters in communities, to let you only display a post to a subset of community members, but everything else works in a comm just like it does in a journal.