At least on current dev (1.67.140215) you can set 'probability of presence' for a group which one would expect to be a nice useful way to have a group appear randomly instead of having to do the 'set-presence-of-leader-then-on-each-member-on-presencecondition-check-if-leader-exists' dance, or script it, or whatever.
But what it does it just sets the condition of presence of each individual group member to be the same which means that each group member spawns randomly, not group as a whole.

This feature doesn't make much sense because even if you used it for randomizing the size/construct of a group there would still be times when the group didn't appear at all???