An electromagnetic particle-in-cell simulation code is used to investigate particle density effects on the nonlinear amplification of inverse-bremsstrahlung electron acceleration, according to the schemes proposed by Kawata et al. and by Hussein and Pato. It is shown that although the space-charge self-consistent field does not modify the basic results obtained with the one-particle model, the electromagnetic fields produced by the transverse particle current play a substantial role for large beam densities.