A guy in a blue shirt chases a guy in an orange shirt during a game of flag football.
Two people are playing flag football.
'During a game of flag football' implies that the guys are playing flag football.
Since guys are people, if one guy chases another guy during flag football, then there are two people playing flag football.
Two guys in different shirts would infer that at leat two people are involved.  During a game of flag football means that they are playing the game.