Hello, I found by myself the way to do what I was looking for with this script:   
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^   
AD1 = GROUP:FindByName( "AirDefense1" )   
RespZoneAD1 = ZONE:New( "RespZoneAD1" )   
AD1:InitZone( RespZoneAD1 )   
Radar3 = UNIT:FindByName( "Radar 3" )   
AD1:HandleEvent( EVENTS.Dead ) 
function AD1:OnEventDead(EventData) 
 if Radar3:IsAlive() == false 
 then 
 AD1:Respawn():MessageToAll( "AD1 Respawned", 5, "ATTENTION" ) 
 end 
end   
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^   
This script takes the Group spawned without Late Activation in DCS so that's why it starts looking for the GROUP. 
But lat's say now I want to change this, I want to put a Late Activation, how should I change this script? I tried removing GROUP:FindByName() and used AD1 = SPAWN:New( "AirDefense1" ) and after that I spawned using AD1:Spawn(). The Group spawns but it doesn't Respawn once Radar1 is destroyed. May be I can't use :Respawn() because so far if I understoopd :Respawn() you use for groups are already spawned by DCS. 
I'm asking this also because if I use AD1 = SPAWN:New( "AirDefense1" ) I can also use other :Init.…() commands. 
I think I'm making a bit of confusion but I stared only few days ago to explore MOOSE and LUA. Sometime I'm confused on how to put toghether the script I write and DCS mission or how to put more scripts together or when to use local in front a variable or not. In many example a lot of time has not been used in other yes... If someone can give some clarification about all this would be appreciated. 
Thanks