Hi,
Server groups are defined in management console. You have to go to management console and select > Server groups> add> follow the instructions.
All the job servers listed with the repo will be displayed and you can create one server group for each repo. For more details refer this link
http://help.sap.com/businessobject/product_guides/boexir32SP2/en/xi321_ds_admin_en.pdf
Arun