Inertial particle microfluidics is a relatively new technology that exploits inertial effects to manipulate and control particle dynamics. Under inertial flow conditions, particle-particle interactions can lead to the formation of pairs and trains of particles. We analyse the behaviour of two and multiple capsules in channel flow under moderate inertia. We employ an immersed-boundary-lattice-Boltzmann-finite-element solver to account for the fluid mechanics, the capsule dynamics, and their coupling. We find that the formation of stable pairs of particles depends on the relative size and softness of the particles and their positions at the time of their first encounter. Suspensions with more particles show shear-induced fluctuations that compete with inertial focussing. Applications relying on lateral focussing, such as separation, or precise inter-particle spacing, such as cytometry, might benefit from our findings.