Create your squad. Give each soldier the probability of presence that you want him to have given that the squad exists. (That is, if the squad exists you want a 50% chance there is a medic, then move the medic's slider to 50%)
Now move the slider of the group leader to the place you want. Name the leader, lets call him Bob. For each of his loons write this in the field called Condition of presence
alive Bob
If Bob is not created then none of the rest of the group will be. If he is created then they will all be there, subject to whatever probability of presence you have given them.
In other words, for a unit to appear the condition line must be satisfied. If it is, the game then looks at the probability slider.
Does that make sense?
Obviously you can put any boolean condition in the condition field, it doesn't have to be anything to do with the group leader.
Oh, and
Welcome to the forums!
I'm surprised you have much time for OFP editing, what with running N Korea from beyond the grave and all. ;D