In this paper, a lightweight horn antenna array demonstrator is designed and fabricated by means of an additive manufacturing (AM) process that has been optimized for complicated passive radio-frequency (RF) components. The presented design comprises a complex waveguide feeding network (WFN) which aims to exploit the increased flexibility offered by this novel fabrication technique. This WFN concept could be easily extended to larger antenna arrays with higher gain for full-size telecommunication applications. The electromagnetic performance of the featured structure is validated by means of full-wave numerical simulations. Fabricated prototypes of the WFN and of the complete antenna array are tested and are found to exhibit a promising agreement with the simulation results.