Split your group into two teams: A throwing team and a fielding team.
The fielders have to put one player by each post and the rest in the field. Players from the other team have 1 ball each.
Each player from the throwing team has to come into the batting square and call out which base they are going to run to. Immediately after call out they must then throw the ball to one of the fielders after which they have to try and beat the ball to the post.
The ball then gets thrown back to the square for the next player to call, throw and run.
Make sure that this games flows by encouraging the fielders to feed back the ball to the next thrower as quickly as possible.