Selecting ungrouped objects

# a little verbose? :)
import bpy

scene_groups = bpy.data.groups

# make sure before this, that nothing is selected.
names_of_grouped_objects = []

for i in bpy.context.selectable_objects:
    for group in bpy.data.groups:
        for item in group.objects:
            if item.name == i.name:
                print("i.name", i.name, " is grouped in ", group.name)
                names_of_grouped_objects.append(i.name)

# ungrouped objects
print("ungrouped objects are these")
for i in bpy.context.selectable_objects:
    if i.name not in names_of_grouped_objects:
        print(">>>",i.name)
        bpy.data.objects[i.name].select = True