Edit the user with Dashboard -> Users -> Authors & Users and flag the user you want to be a member with the option at the bottom.
Inside posts, add [members] content you only want members to see [/members]
When setting a 'members' category, you *cannot* use an existing comic category, uncategorized, or blog category!
You MUST create a whole new category and called it "members", then you select that category here and create a page called "Members" or something equivelant and associate the Member's Only template to it.
This will make it so that that category is only seen as blogposts inside that area and not anywhere else on the site unless the user has the members flag.