Adding tests for #27467 surfaced some bugs:
from pandas import *
df = DataFrame(dict(a=[0, 0, 0, 1, 1, 1], b=range(6)))
g = df.groupby(np.repeat([0,1], 3))
# ok
print(g.ngroup())
#fails
g.transform('ngroup')
AttributeError: ("'Series' object has no attribute 'ngroup'", 'occurred at index a')
Comment From: mroeschke
This looks to work on main and looks like we have tests for this so closing