You can transform a default, single weblog WordPress installation into a multi-blog site with the help of PostGroups.
Once the first group is created, PostGroups will change your blog home page to feature only the latest posts from each group you add to your blog, ordered by their date. If you want to see all posts from a group, you need to navigate to that group from your blog main menu or from the PostGroup widget.
PostGroups will append the groups to the list of pages as returned by the
wp_list_pages function, should you choose to do so in the options page for the plugin. However, if your theme is using a different method to retrieve and display your pages, then you need to use the provided PostGroups widget to show the groups.
All your posts that are not assigned to an existing group will go to a default group called 'Ungrouped posts'. If at some point you forget or don't want to assign a post to a group, then the plugin will automatically show this default group (the name used by this group can be set in the options page for PostGroups).