  We determine the possible finite groups $G$ of symplectic automorphisms of
hyperk\"ahler manifolds which are deformation equivalent to the second Hilbert
scheme of a K3 surface. We prove that $G$ has such an action if, and only if,
it is isomorphic to a subgroup of either the Mathieu group $M_{23}$ having at
least four orbits in its natural permutation representation on $24$ elements,
or one of two groups $3^{1+4}{:}2.2^2$ and $3^4{:}A_6$ associated to
$\mathcal{S}$-lattices in the Leech lattice. We describe in detail those $G$
which are maximal with respect to these properties, and (in most cases) we
determine all deformation equivalence classes of such group actions. We also
compare our results with the predictions of Mathieu Moonshine.
