Create a User table with two fields, UserName and the calculation field CountJobs. Connect the User table with your Jobs table using the relationship Jobs::AssignedName = User::Username. Then make CountJobs = Count(Jobs::_kp_Jobs). You don't have to use the primary key (_kp_Jobs), any field that is guaranteed to be non empty will work.
User::CountJobs will tell you the number of jobs for each user.
Ah yes, I see that if I am to do this in real time, I need to use relationships.