By creating a group that has access to all Site you can have a central Help Desk that provide support to all end users 


In the example below we have a central Help Desk and want all Help Desk users to be able to access all sites, but we assign tickets for each site to a specific Help Desk user except for Boston which has all tickets assigned to all Help Desk Users


Bill Covers New York



Jamie cover Los Angeles



Jane covers Chicago



All three cover Boston