You could try using the 'stop' and 'dostop' commands:
leader group1 stop true
Or maybe something like this:
group this stop true
The second code can be put in the group leader's init line.
It's been awhile since I last scripted anything for ArmA, but hopefully those commands will work.
Surprisingly, the 'stop' and 'dostop' commands are not in the 'ArmA only' comref, despite those commands being present in the game.
Hope this helps