Before revision:  Colonies of aligned pusher particles are shown to elongate in the direction of particle orientation and undergo a cascade of transverse concentration instabilities . Colonies of aligned puller particles instead are found to elongate in the direction opposite the particle orientation and exhibit dramatic splay as the group moves into the bulk. A linear stability analysis of concentrated line distributions of particles is performed and growth rates are found, using an active slender-body approximation, to match the results of numerical simulations. Thin concentrated bands of aligned pusher particles are always unstable, while bands of aligned puller particles can either be stable (immotile particles)  or unstable (motile particles) with a growth rate which is non-monotonic in the force dipole strength.
After revision:  Colonies of aligned pusher particles are shown to elongate in the direction of particle orientation and undergo a cascade of transverse concentration instabilities , governed at small times by an equation which also describes the Saffman-Taylor instability in a Hele-Shaw cell, or Rayleigh-Taylor instability in two-dimensional flow through a porous medium. Thin sheets of aligned pusher particles are always unstable, while bands of aligned puller particles can either be stable (immotile particles) or unstable (motile particles) with a growth rate which is non-monotonic in the force dipole strength.