Need help relating a list of random numbers to a list of names so that i can create a rota

But given that there are 30 distinct duties and only 22 staff members, i would have to devise a formula to randomly determine who the 8 additional staff members were. This is something i have been struggling with - i have also found it tremendously difficult to 'translate' that kind of information into anything resembling a staff rota. I don't know how to relate the unique random numbers to the names/tasks to allow the creation of an instantaneous rota. I'm obviously missing something... I have attached what i have done thus far.