How do you program a new keyless entry key fob on a 2008 Pontiac Grand Prix?

You should be able to look this up in your manual. If push comes to shove you might have to go back to the dealership and have them do this. Some are harder than others to program. In this instance you want it done right.

How do you program a new keyless entry key fob on a 2008 Pontiac Grand Prix?
Sorry to say but the owner's manual hasn't got any information that would help to program the remote keyless entry fobs.
The nearest Pontiac dealer will be able to connect the factory scan tool to your vehicle and program those remotes for you. Some have a nominal charge for this but others will do this fairly simple programming as a courtesy. Call around for estimates.
Have all remotes for this vehicle together when you go to get the programming done.
I hope this helps.