  We investigate theoretically the collective dynamics of soft active particles
living in a viscous fluid. We focus on a minimal model for active but
non-motile particles consisting of $N>1$ elastic dimers deformed by active
stresses and interacting hydrodynamically. We first derive a set of effective
equations of motion for the positions of the particles. We then exploit these
equations in two experimentally-relevant cases: uncorrelated random internal
stresses, and uniform monochromatic external shaking. In both cases, we show
that small groups of intrinsically non-motile particles can display non-trivial
modes of locomotion resulting from the hydrodynamic correlations between the
particle-conformation fluctuations. In addition, we demonstrate that a coherent
shaking yields spatial ordering in suspension of soft particles interacting
solely through the fluid.
