BPM user role access

Hi Experts,

Who all can access the BPM process? Is it possible to set the userrole access to the BPM process?

Scenario :

Through webservice, I have to call BPM process..But the BPM process should be accessed by particular user.

For example, Manager related BPM process should not be accessable to the Developers.

How to set/give the BPM user role access?

Regards

Sara