If you have a General with say... 2 spies against an enemy army of 6 without any Scouts, then the best way to bribe units without them being harmed is to bring your General with your Spies. Before you start bribing, have your General use the 'Ambush' ability. Once you've successfully bribed your units, they will be temporarily invisible within the General's radius. As long as they're not attacking anything, the enemy units will not see them, along with your Spies (if you're not Russian).