Does it work in preview mode when you are creating the mission?
Yes
Set the leader with a particular probability of presence. Call him Grp1Leader. Then for each of the other units in the group you need to set the condition of presence as: alive Grp1Leader
If you don't do that last step then the other guys will aways be there.
That will give you a whole group that may or may not be there. You could also give each unit a probability of presence and so get groups of varying size.