arcgis desktop - Auto incrementing field based on groups within feature class?

I am needing to auto increment a field based on groups within a feature class. I have 8 plots within a given polygon and I need to assign them an ID from 1-8 for each set of plots within each polygon. The polygon would have its own unique ID number to be used to group the plots.

I assume it would be an alteration of this:

rec=0
def autoIncrement():
 global rec
 pStart = 1 
 pInterval = 1 
 if (rec == 0): 
  rec = pStart 
 else: 
  rec = rec + pInterval 
 return rec

Answer

Field calculator for Python

d={}
def GroupOrder(groupID):
  if groupID in d: d[groupID]+=1
  else: d[groupID]=1
  return d[groupID]

GroupOrder( !locality! )

Change !locality! to relevant field.

UPDATE: This variation of expression:

d={}
def GroupOrder(groupID):
  N=d.get(groupID,0);N+=1
  d[groupID]=N
  return N

Should work much faster on large datasets.