If you change your mind and want to create unique groups for sharepoint site after it was created there will be an issue because "Setup Groups for this Site" disappeared. Even you can create groups and make them to be default groups for the site but "Add users to Sharepoint Group" will not be available from drop down box of "your own" Site Owners group.
Luckily you can still get "Setup groups for this site" page appeared by this url
siteurl/_layouts/permsetup.aspx
Hope it will help
Cheers, BB
