Actually, thresholds diminish the power of magic inside the entire building, not something crossing from the outside to the inside. So if a Ward works like a threshold, it does exactly that; reduce powers of those people not invited.
I'm not sure where you're getting that. Every reference I found refered to crossing, not being within. A few examples from the section on thresholds (YS230):
"the word threshold is used to describe the
barrier that is formed around a home"
"In the broadest sense, the term “threshold” may be given to any metaphysical
barrier that impedes or blocks supernatural power from
passing from point A to point B." <-- (This basic definition seems extremely clear)
"Some beings—especially those of pure spirit—cannot
cross a threshold [...] If they
cross a threshold without counteracting it"
"when an entity
crosses a threshold"
"
crossing a threshold doesn’t make the wizard melt"
"a wizard
crossing a threshold uninvited leaves a large amount of his supernatural power"
"Even a spell that tries to
cross a threshold may be diminished."
"Many spell effects and some supernatural abilities will erode or completely disappear if they are
carried across a threshold"
"such entities are particularly vulnerable to
crossing thresholds. They may actually take damage from
crossing one"
Please direct me to any place in the rules where it states anything about what effect a ward might have on anything already inside? The only related reference I could find is the protective wards used when summoning, which keep the demon from escaping but otherwise have no impact on them.