Title: A Coach-Player Framework for Dynamic Team Composition. Abstract: In real-world multi-agent teams, agents with different capabilities may join or leave "on the fly" without altering the team's overarching goals. Coordinating teams with such dynamic composition remains a challenging problem: the optimal team strategy may vary with its composition. Inspired by real-world team sports, we propose a coach-player framework to tackle this problem. We assume that the players only have a partial view of the environment, while the coach has a complete view. The coach coordinates the players by distributing individual strategies. Specifically, we 1) propose an attention mechanism for both the players and the coach; 2) incorporate a variational objective to regularize  learning; and 3) design an adaptive communication method to let the coach decide when to communicate with different players. Our attention mechanism on the players and the coach allows for a varying number of heterogeneous agents, and can thus tackle the dynamic team composition.  We validate our methods on resource collection tasks in multi-agent particle environment. We demonstrate zero-shot generalization to new team compositions with varying numbers of heterogeneous  agents. The performance of our method is comparable or even better than the  setting where all players have a full view of the environment, but no coach. Moreover, we see that the performance stays nearly the same even when the coach communicates as little as 13% of the time using our adaptive communication strategy.  These results demonstrate   the significance of a coach to coordinate players in dynamic teams.