try to place the flash as high as possible, if the light coming from bottom, the person in front will cast shadow on the person behind.
and btw, are u using IR or RF for wireless trigger? IR trigger may not work under bright sunlight and u need to place your master flash behind the slave so that the slave can sense the signal.
if you place the slave flashes in a semi-circle, the exposure will be uneven. but if you put them in a straight line, the slave may not be able to sense the signal esp in the day (I have tried and failed)