Semi-Private Posts?

Hi all,

I’m currently working on a site for a client and they want something quite specific when controlling who sees posts.

In general it is a private site but on the homepage they want to be able to display snippets of the post but when someone clicks through to the post they need to be presented with a login (if not already logged in) in order to view the whole post.

So I guess my question is:

1. Is it possible to use the native WordPress users feature for this or will I require some kind of plugin?

Any help would be much appreciated.