It is possible to assign people in the same role to different charge-out rates.
You can set up a role for each person and set a Charge-out Rate specific to that person.
Go to Manage > Roles, click the New Role button
Name the new role as say "Designer - Amy" and add the default charge-out rate and cost. The cost should equal the cost in Amy's contract.
Add another new role say "Designer - Peter" and add the charge-out rate for Peter.
Now you can see that Amy and Peter have different Charge-out rates:
Go to Manage > People, add Amy and Peter and set their respective roles as "Designer - Amy" and "Designer - Peter"
Then in a project, you can add Amy and Peter in their roles and Runn will use their different charge-out rates to do the calculations.