The solubility of AgI in NaI solution is less than that in pure water because of common ion effect.
$\mathrm{A}\mathrm{g}\mathrm{I}\left(\mathrm{s}\right)\rightleftharpoons \mathrm{A}{\mathrm{g}}^{+}\left(\mathrm{a}\mathrm{q}\right)+{\mathrm{I}}^{-}\left(\mathrm{a}\mathrm{q}\right)$
The common ion, ${\mathrm{I}}^{-}$ , obtained from strong electrolyte NaI shifts the equilibrium from left to right and this results in the decreases of solubility of AgI.