  In this paper we investigate relations between Koopman, groupoid and
quasi-regular representations of countable groups. We show that for an ergodic
measure class preserving action of a countable group G on a standard Borel
space the associated groupoid and quasi-regular representations are weakly
equivalent and weakly contained in the Koopman representation. Moreover, if the
action is hyperfinite then the Koopman representation is weakly equivalent to
the groupoid. As a corollary of our results we obtain a continuum of pairwise
disjoint pairwise equivalent irreducible representations of weakly branch
groups. As an illustration we calculate spectra of regular, Koopman and
groupoid representations associated to the action of the 2-group of
intermediate growth constructed by the second author in 1980.
