A Traitor Within [Passive] Gríma can be deployed in one of two way, though his controlling player must decide which before either Army is deployed. Gríma can either be deployed as part of Saruman's Warband (in which case he will not take up a space in the Warband), or as part of the enemy Army.
If Gríma chooses to be deployed as part of the enemy Army, he must select an enemy Hero. Once that Hero and their Warband has been deployed, Gríma's controlling player can deploy Gríma as if he was also part of that Warband, following all the usual deployment rules. Gríma is still part of his controlling player's Army.
If Gríma is deployed as part of the enemy Army, then enemy models treat him as a friendly model, and therefore cannot shoot him, Charge him, make Strikes against him, target him with special rules, or make a shooting attack in which Gríma would be In The Way. They may also ignore his Control Zone. They can, however, target him with Magical Powers any other enemy model.
Whilst Gríma is considered a friendly model, Gríma may ignore the Control Zones of enemy models and may Move through enemy models as if they weren't there. When Gríma Moves this way, he cannot end his Move overlapping or in base contact with an enemy model. Gríma can choose to Charge and enemy model if he wishes.
These restrictions immediately end if Saruman is slain, it Gríma Charges an enemy model, if Gríma destroys an enemy Siege Engine, or if Gríma interacts with an Objective Marker (such as destroying a Supply Marker or picking up an objective), at which point Gríma is treated as a normal enemy model and can be Charged, shot as and slain like any other model.