If you want to set a strict priority between users or groups, you can use the fairshare tree without turning on fairshare (although turning on fairshare is more fair). You create a resource_group file. If you want to set priorities between unix groups, set fairshare_entity to egroup otherwise leave it as euser. The names in the resource_group file will be user/group names. Lastly set job_sort_key to fair_share_perc or use the fairshare_perc term in your job_sort_formula.
The admin guide provides a little more detail. Look at the 14.1 admin guide section 4.8.14.