Press the New button underneath the scroll bars, name the custom group appropriately, and then move the journal of the person who you want to be able to see the post from the Not in group: box to the In group: box. You do this by clicking on the name, then pressing the >> buttons. Once that is done, press Save changes.
Then, when you make your post, choose the option of Show this entry to:Custom.... Click the option of the custom group you just made, and post!