If you refer participants as the user of workbench which are part of a particular process you can surely do it. Make a List variable in the process which will store emails. Now append all participant's mail addresses to this variable for each user. Now when the process reaches the last step (just before completion), loop through the list and send a notification mail to all participants.
Another way is to create a group which has all the participants of a particular process. When process reaches to the last point get participant's mail ids using user lookup service. Send mail.
Hope it helps!