Or you could have a loop like this :
_group=_this select 0
_i=0
#loop
_num=(count units _group)
?!(alive (units _group select _i)):hint format ["Oh no ! %1 is dead", name (units _group select _i)];[units _group select _i] join grpnull
_i=_i+1
?_num==0:exit
? _i>= _num :_i=0
~0.001
goto "loop"
Implementation : [_mygroup] exec "casualties.sqs", in a trigger with "true" in the condition field, for instance...
*edit*
Instead of "hint", you could use :
leader _group sidechat format ["Oh no ! %1 is dead", name (units _group select _i)]