No no no no no no no no no no no no no.
Read the comref again. And this time read it properly.
(Never, ever, under any circumstances whatsoever, make a post in the forum or indeed read a post in the forum without looking up ALL commands in the online comref. You may use other versions of the comref for quick reference, but not using the online version in the Editors Depot the first time you encounter a command is merely stupid. In fact it is the ONLY version of the comref I ever use, because you never know what help somebody may have added since the last time you looked.)
Done that? Good. Now you understand that it is impossible for the group leader to be dead. If the group leader is dead, then by definition everybody in the group is dead. You MUST understand what a group is, and the difference between a unit and a group (which are barely even related) otherwise you are never going to get anywhere in OFP editing. Ask if you are unsure: that is what the forum is for.
Oh and yes of course you can use forEach in the Condition field of a trigger, in the same way that you can use virtually any command in virtually any circumstance where a command can be put.