Hi,
If each user / buyer is having own purchase group, then you can go for authorization method with authorization object : M_BANF_EKG. You may check the feasibility with BASIS team.
Else, you cant achieve it by standard SAP, as of my knowledge,. You may go for development using the BAdI: ME_PROCESS_REQ
You may refer the similar thread: PR creator should only chnage the PR
Regards,
AKPT
