Hi Oliver, I have done this with some of our processes and the only way I managed it was to create a dummy support group, add the end users to this support group (making sure they receive group emails), have the request assign to this group at the point when approval is required and ensure that the "Norify Assignee" is ticked on this assignment. This assignment will have to have a link in so that the end user can approve via the service portal and you need to give the role you have created permissions to this action.
Also, you need to make the process move on at the point of Approval to assign to the relevant support group required to carry out the task.
Hope that makes sense....
Nic