Let's say army A has 5 stalkers and army B has 5 stalkers and a sentry.
If the sentry is in the middle of army B and throws up a guardian shield while army A is still well outside the radius of the shield, obviously army B will take reduced damage from A's ranged attacks.
If the A stalkers sacrifice the time and health to walk inside of the guardian shield, literally standing right next to B's stalkers, are their attacks still reduced by the sentry's shield?