For a selected object, create an array of instances of the object's group.

add_group_instance_array.py

import bpy
from itertools import product
from mathutils import Vector

def get_enum_items(self, context):
    obj = context.active_object
    groups = obj.users_group
    group_names = [g.name for g in groups]
    return [(gn, gn, 'Instance group "%s"'%gn) for gn in group_names]

class OBJECT_OT_add_group_instance_array(bpy.types.Operator):
    """For a selected object, create an array of instances of the object's group."""
    bl_idname = 'object.add_group_instance_array'
    bl_label = 'Add Group Instance Array'
    bl_options = {'REGISTER', 'UNDO'}

    count = bpy.props.IntVectorProperty(
        name = "Count", subtype = "XYZ", default = (1, 0, 0), min = 0,
        description = "Object count along each axis.")

    offset = bpy.props.FloatVectorProperty(
        name = "Offset", subtype = "XYZ", default = (1.0, 1.0, 1.0),
        description = "Object coordinate offset along each axis.")

    group = bpy.props.EnumProperty(
        name = "Group", items = get_enum_items,
        description = "Group to instance.")

    @classmethod
    def poll(self, context):
        return context.active_object != None \
            and len(context.active_object.users_group) > 0

    def execute(self, context):
        obj = context.active_object
        group = bpy.data.groups[str(self.group)]
        center = group.dupli_offset
        
        count_fun = lambda x: range(x+1)
        count_product = list(product(*map(count_fun, self.count)))[1:]
        for count in count_product:
            loc = center + Vector([count[0] * self.offset[0],
                                   count[1] * self.offset[1],
                                   count[2] * self.offset[2]])

            dup_obj = bpy.data.objects.new(group.name + '_array.000', None)
            dup_obj.location = loc
            dup_obj.dupli_type = 'GROUP'
            dup_obj.dupli_group = group
            context.scene.objects.link(dup_obj)

        context.scene.objects.active = obj

        return {'FINISHED'}

def register():
    bpy.utils.register_module(__name__)

if __name__ == "__main__":
    register()