We address the question to know which are the minimal ingredients to obtain a decentralised grouping of agents that move on grid according to discrete local rules. The agents and their environment are described with a stochastic discrete model inspired from an example taken in biology: the aggregation of the cellular slime mold {\em Dictyostelium Discoideum}. The environment transmits information according to a reaction-diffusion mechanism and the agents move by following excitation fronts. We show that despite its simplicity the model exhibits interesting properties of self-organisation. Moreover, first observations show that the system is robust to introduction of noise on the moves of the amoebae and robust to the introduction of obstacles on the grid.