Use the
createUnit command, and replace the unit in the example with the cfgVehicle of your choice. A complete listing of all cfgVehicles in ArmA can be found in OFPEC's excellent
Comref.
EDIT: Here's an example. Go to the editor, place a radio trigger on the map and write this in the "on activation" field:
mySoldier = group player createUnit ["SoldierWB", position player, [], 0, "FORM"]
Activate the trigger and you will have a BLUFOR Rifleman in your group. To see other units, simply replace the SoldierWB-part of the code above with any cfgVehicles-entry from the list I linked to.