  We include alignment interactions in a well-studied first-order
attractive-repulsive macroscopic model for aggregation. The distinctive feature
of the extended model is that the equation that specifies the velocity in terms
of the population density, becomes {\em implicit}, and can have non-unique
solutions. We investigate the well-posedness of the model and show rigorously
how it can be obtained as a macroscopic limit of a second-order kinetic
equation. We work within the space of probability measures with compact support
and use mass transportation ideas and the characteristic method as essential
tools in the analysis. A discretization procedure that parallels the analysis
is formulated and implemented numerically in one and two dimensions.
