Sorting Activity Code groupings by the start date of the activities within

to sort within a group just click on the top of the start column

also under group there is an option for sorting. ON the right select add then seleect start date

The other option is to give an chnage the of the grouping itself by chaging the order with in the activity code dictionary 

another way is in the grouping to do not select summarize instead select view then summarize to level and pick a level that you want to summarize to

Take a look at the following link, it might give you some clues.

How to Group and Sort By Dates | P6 Consulting