Quoting TheRealWarpstorm, reply 27
Each Zeus grants HPs to all units in the Army.
Was this spelled out somewhere? I did not know this.
Remember this is only when the unit is part of an Army (created with V), not if it is un-attached.
I thought it only gave a bonus to the T3 though and not the whole army, can anyone confirm either way?
EDIT: Ooops, The quote was from TheRealWarpstorm, a Dev, so I guess that is a confirmation right there 
That does make the boosts very powerful, I had not realised. I guess it makes sense because it also encourages creating a meta-unit even without a T3.