  We develop a method for computing correlation functions of twist operators in
the bosonic 2-d CFT arising from orbifolds M^N/S_N, where M is an arbitrary
manifold. The path integral with twist operators is replaced by a path integral
on a covering space with no operator insertions. Thus, even though the CFT is
defined on the sphere, the correlators are expressed in terms of partition
functions on Riemann surfaces with a finite range of genus g. For large N, this
genus expansion coincides with a 1/N expansion. The contribution from the
covering space of genus zero is `universal' in the sense that it depends only
on the central charge of the CFT. For 3-point functions we give an explicit
form for the contribution from the sphere, and for the 4-point function we do
an example which has genus zero and genus one contributions. The condition for
the genus zero contribution to the 3-point functions to be non--vanishing is
similar to the fusion rules for an SU(2) WZW model. We observe that the 3-point
coupling becomes small compared to its large N limit when the orders of the
twist operators become comparable to the square root of N - this is a
manifestation of the stringy exclusion principle.
