You will need
a cycling helo waypoint, which is stopped cycling by group lockwp true or some other boolean
you will need a land script (probably)
and you will probably have to create a unit in the group, somewhere out of harms way and disableai "move" him to stop the unit trying to move and hjoin the soldier group (This is to maintain the soldier group name)
Each soldier in the group would need a killed event handler attached to them, so that you woul;d knoiw when the respawn numbers reached a limit before calling in the helo transport, or alternatively a trigger at the respawn point
this trigger or killed event script would check the number of units waiting to be transported and then unlock the helo waypoint, allow the units to movein and off it goes on its reinforcement run
very basic outline