Hiding a unit does not remove it from the game. The unit will still autotarget enemies and attack/move. They won't be able to retaliate.
To efficiently make what you need create a unit and give it the enviroment model that blends, in this case the bush,high grass or whatever. Make the unit unselectable (locust, -1 to selection circle, or a simple trigger to unselect it when clicked uphon).
So:
1. Make grass-unit
2. Make it unselectable
3. Create a spell based of the Sorceres's invisibility.
4. Make a trigger when a unit comes in X range of the grass, cast invisibility on it. Duration of invisibility should be a lot.
5. Make a trigger when a unit leaves the range of the grass to remove the invisibility buff.
6. Have fun
p.s. To be able to see other invisible units while in bush , create a dummy with true sight the same way as above. Give it to entering player, kill it when he leaves the bush. (this might cause some problems if you got a invisible hero (e.g. twitch like) entering the bush while stealthed)