dataDict = {'a':['one','two'], 'b':[[2,3],[1,3,6]], 'c':[1,2]}
dataDf = pd.DataFrame(dataDict)
df = t.groupby(['a', 'c']).b.apply(lambda x: pd.DataFrame(x.values[0])).reset_index().drop('level_2', axis = 1)
df.columns = ['a','c','b']
df['d'] = df['b'].astype(str) + ','
df.groupby(['a','c']).d.sum().reset_index()