Adding mesh to previously rigged object I have a scene that includes an appended rigged hand model that I built previously. For this specific scene, I want to add press-on nails to the hand.

I joined the the mesh of the nails to the hand mesh, but when i go to move the rigging, the fingers move independently of the nail. Is there anyway to make the nails move with the fingers with the pre-exsiting rig? Or do i have to delete the armature and re-rig the hand after joining it to the nails?

I also don't want the nail to be deformed by the rig. How can I avoid this?
You can add geometry after rigging without any need to delete your previous job.
If you used the automatic weights option, probably you will have to manual weight the new vertices: as Mentalist said, assign to the fingertip bone a weight of 1.0, but also check if there is some weight assigned to any other bone: if so you get deformations and you should remove these assignements.
In edit mode, if you select a vertex you can read wich are all his weight assignements in the object properties panel (Hotkey N).