I only wanted to restrict access to a couple of pages but in the end I had to make every other page editor only, then restrict the three pages I wanted to share editing with accessible by 'author' only.
Would have been handy to have every page defaulting to admin only, then I could have just selected 'editor' or 'author' for the shared pages after creating a group.
So what I did was create one group with 'editor' or above access, then made every page in the editor group. Then I created another group 'author' and shared the three pages with users of 'author' access so that they could edit them.
Unless I've just been silly and created a lot of work for myself!!!! But the other pages defaulted to 'all' which defeated the point of creating a group I thought.