Find relevant rows falling under a date range in Pandas Dataframe and do some aggregations for these rows

Question

I have 2 Pandas dataframe as below:

A = pd.DataFrame({\'UserId\':[\'U1\',\'U1\',\'U1\',\'U2\',\'U2\',\'U3\'],\'Start_Week\':[\'1/6/2020\',\'15/6/2020\',\'6/7/