The term user is not used within the IPT context. A participant is a person who carries out the information processes within an information system. The term user falls under the category of a participant as a participant is any person who has some sort of input into the IS system. As a participant can be someone who carries out regular checks to the system right down to the person that logs onto the network -- what you are wrongly calling a user instead of a participant.
I hope that I have clarified this for you. If you are confused by my explanation refer to your textbook or ask your teacher to go over it with you.