How do you iterate through groups in a pandas Dataframe, operate on each group, then assign values to the original dataframe?

    yearCount = df[['antibiotic', 'order_date', 'antiYearCount']]

    yearGroups = yearCount.groupby('order_date')

    for year in yearGroups:
        yearCount['antiYearCount'] =year.groupby('antibiotic'['antibiotic'].transform(pd.Series.value_counts)

In this case, yearCount is a dataframe containing 'order_date', 'antibiotic', 'antiYearCount'. I have cleaned 'order_date' to only contain the year of the order. I want to group yearCount by the years in 'order_date', count the number of times each 'antibiotic' appears in each "year group" then assign that value to yearCount's 'antiYearCount' variable.