Hello,
i advise you to check /SAPSRM/BD_WF_SC_RR_PGRP BAdI implementation in order to understand how is managed:
- area type (instance of /SAPSRM/CL_WF_AREA_XXX to determine the responsible approvers),
- leading object (in your case, it cannot be the pruchasing group...)
- area guid.
It does not seem you have to copy/paste standard ABAP code (which is used for a precise business need which is not yours...).
Regards.
Laurent.