Dispersion cannot be computed for individual data. You must define subpopulations and this is what the AGGREGATE option allows you to do. See this usage note on overdispersion: http://support.sas.com/kb/22/630.html and the link within it to a note on defining subpopulations.
The answers to many questions can be found in the Samples and SAS Notes in our searchable knowledgebase, http://support.sas.com/kb. You can use the search engine there to find the answers you need.