This add-on gives you the ability to restrict Alfresco Share site creation to a specific group of users. Users not in the group will not see a "Create Site" link in:
The header Sites dropdown menu
The My Sites dashlet
The "welcome" dashlet
In addition, for users not in the group, the "welcome" dashlet's text changes to explain what a Share site is but does not imply that they have the ability to create one. A nice enhancement might be a link that launches a workflow to request a new site. (Pull requests welcome!)
By default, the group the module looks for must have an ID of "GROUP_SITE_CREATORS". The display name can be anything.
This add-on also changes the low-level permissions so that even if someone figures out how to create a site without the user interface, the repository tier won't let them do that unless they are in the group.