You are mixing two different (similar) things.
1) Probability of presence = a power bar, where you can
set a percentage for the probability of presence.
This means, if you set it to 50%, there's a 50/50 chance,
that the unit will be present, when the mission starts.
2) Condition of presence = a boolean, to decide wether
the unit will be (or not) present at the start of the mission.
:note - you can't combine the condition of presence for
a unit of group1 with the presence of a unit from another
squad (at least until 1.46 - not tested in 1.75 yet).
You can use e.g:
group leader: set the probability of presence to 50%
and put into each soldier's condition of presence field
(soldiers of this squad only):
alive leader_name
In both cases, you can't use it to let units appear later
in the mission.
If you want to let appear units at certain events, then better
use the createvehicle command, or a simple setpos/getpos.
hope this clears things
~S~ CD