Yes, very well possible.
I suppose you will manually confirm your friends to be realy friends.
So you place them manually into a group.
In the backoffice, go to the tab 'users' and click on the user you want to put in a group.
Then choose the group you want that user to be in. (it is the same group the group I'm speaking off in the next point)

Give that group 'member' rights to the blog(s) that you are posting in.
In the backoffice, click on blogsettings, and then choose the blog you will post in (probably all of them)
Click on group permissions and then choose for that group 'member' and hit save!.

Make sure you only write 'protected ' posts
In the file conf/_admin.php on line 47 (more or less) you change this

$default_post_status = 'published';

to

$default_post_status = 'protected';

just to be sure you won't make an error

From this moment on, you only publish protected posts, that only members can read.
Since you put your registered friends in a group that will allow membership, that is what you want, right ?

Thank you very much!
Your help is usefull, however i want a bit more privacy, Is it possible somehow to not too show blog name, users online and Categories name for Unregistered users? or i should probably make a php script with a form to login ? maybe it already exist?

add this to conf/_basic.config.php ( just after the <?php line ) :

$login_required = true;

Anyone who's not logged in will be taken to the login screen and won't be able to see any of your blog(s) until they login ;)

¥

That's what i was looking for!!! Thanks!