The addition of NaCl to AgCl decreases the solubility of AgCl, because
Ksp of AgCl decreases
Ksp of AgCl increases
Solution becomes unsaturated
Ionic product exceeds the Ksp value
Condition for precipitation Kip > Ksp. NaCl is a strong electrolyte and so it will furnish a high concentraiton of Cl- ion in the resultant solution.