=>The mock up is quite neat and self explanatory, it seems to be a generic solution though, as it does'nt specifically talk about the module being a part of Desktop or web app. Solutions can vary based on that knowledge.
=>Using Pop or an extended side panel is not a bad solution but in this particular scenario since you have only 3 items to be entered/selected by user, you can very well use the original space in the column 4 (Roles) of the data grid. Once a user clicks edit, that particular cell should expand so that the user can select the role, date and level somewhat like this -
COLUMN 4 ............................................................. COLUMN 5
Role Drop down ...... Date Picker
Level Slider
=> Also, you could use icons for save,edit& delete instead of big buttons that unnecessarily use up more space.
Hope this helps...
Regards,
Supriya