There's currently no way of grouping users in any way, however, there is a way to limit the user list in the templates by their security access.
So if you set up your users' security such that they only have access to projects based on regions, then when you create an item for a project, only users who have security access to that project will show up in the list.
This option is under Tools -> System Options -> User Lists Settings -> List users by project access
Dan Suceava
http://www.axosoft.com