grouping doubt

Hi ,
I am new to base sas trying to learn on my own ,
I have a doubt , I have a data set which contains the information about the firms , they start from 1991 to 1996 i want to group them into two groups first one which will start from 1991 and end at 1996 with data being present for every month for each year and second one where there the firm discontinues (ie it stops reporting the data) there are two tasks
Group 1: firms that continue in the sample up until December 1996.
Group 2: firms that disappear in the sample prior to December 1996(identify which month and date, they disapper).