You need to have a readymade group in editor.. at least that's what I think.
Whenever (a lot) I use createunit I place a single unit in editor to some remote location and then in a script setpos him to where I want the group to be when I start creating more units to it.
For example
#Stsedlo
?_Stsedlo == 1: goto "Okrouhlo"
?(leader maingroup) distance Stsedloman > 900: goto "Okrouhlo"
_Stsedlo = 1
?(local server): "SoldierWCrew" createunit [getmarkerpos "Stsedlo", Stsedlogroup, "",_skill]
?(local server): "SoldierWCrew" createunit [getmarkerpos "Stsedlo", Stsedlogroup, "",_skill]
?(local server): "SoldierWCrew" createunit [getmarkerpos "Stsedlo", Stsedlogroup, "",_skill]
?(local server): "SoldierWCrew" createunit [getmarkerpos "Stsedlo", Stsedlogroup, "",_skill]
?(local server): "SoldierWCrew" createunit [getmarkerpos "Stsedlo", Stsedlogroup, "",_skill]
?(local server): "SoldierWCrew" createunit [getmarkerpos "Stsedlo", Stsedlogroup, "",_skill]
?(local server): "SoldierWMortar" createunit [getmarkerpos "Stsedlo", Stsedlogroup, "",_skill]
?(local server): "SoldierWMortar" createunit [getmarkerpos "Stsedlo", Stsedlogroup, "",_skill]