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