Add another phone to Find my iPhone app?

I asked a similar question in another forum, but my situation has somewhat changed.

Basically my wife and I want to keep an eye on our step son who seems to not want to follow any rule we lay down. I managed to talk my sister-in-law out of her 3GS to replace his 3G (so I can install iOS 5 on his "new" phone). But the real reason is that I want to enable the Find my iPhone app so that we can know where he is via GPS.

Now when I fire up the app on my iPhone 4 it has my phone and my MBA that come up. My question is this...how do I add another phone onto my phone?

I mean, literally describe the process to me step by step.

Do I need to install this app on every phone that I want to keep track of? If not, does every phone I want to keep track of need a mobile me/iCloud account? Do I?

Sorry for all the questions, but can't seem to find how this thing is supposed to work exactly.

Or if this question has already been asked (and I have to believe it has), please direct me to that thread as I did a search and was unable to come up with an answer.

Perfect. I couldn't have asked for a better response. And there's no way I could ever have figured that out myself.

Thank you so much!

Actually...I might have outsmarted myself. I followed these steps and was able to get the device to come up in my Find my iPhone app. Ok, great. But I then took the phone up to the step son where he put in his SIM card (and we removed the one from the 3Gs) and he then proceeded to transfer the information from his iTunes account to the new phone. Now everything works, his phone, music, contacts, text, etc. but I can no longer see his device in my app.

Would the switching (or removal) of the SIM card make that happen? And if so, I assume it's just a matter of getting back into his phone and doing the steps listed in the first reply again?

Ah, I see. So I'll still need to go reclaim his phone at some point and redo what I'd done earlier? Well the good news is that it's an easy process. The hard part is getting his phone without him knowing - he practically sleeps with that thing!

Thank you though, for all your help.