This question has crossed the mind of almost every iPhone user at least once: "How do I copy this iPhone app I just paid for over to my wife's, child's or brother's iPhone so that he can use it also?" It can be done! This process will also work to copy any licensed iTunes media from one computer to another. The process you are going to follow is to copy the app from your iPhone to iTunes and then to the other (aka "target") iPhone.
Open iTunes on the computer that is used to maintain the other iPhone.
Note: Make sure you are using the iTunes that belongs to the target iPhone. If you sync to her iPhone from your iTunes, you could potentially mess things up for her and yourself.
Authorize iTunes to use your account information (different from the account that is currently on the computer). This can be done on up to five separate computers. To do this, go to "Store > Authorize Computer."
Enter your iTunes account credentials and click "Authorize." If you have authorized too many computers, it will let you know and you will have to free one up by going to a copy of iTunes you've already authorized and choosing "Store > Deauthorize Computer."
Connect the iPhone that has the licensed applications or media you want to transfer on it to iTunes (aka "your" iPhone). Connect the cable from your iPhone to the computer, and it should show up in iTunes.
Right-click the iPhone icon in iTunes and choose "Transfer Purchases." Once the iPhone is connected, it might ask you to update, restore or back up. Do not allow it to do any of these things.
Your iPhone will say "Sync in Progress," and you'll see the files being copied in the iTunes window. This process could take a long time, depending on how many purchases you have on there.
Eject your iPhone from iTunes when the transfer is complete.
Connect the target iPhone to the computer. It should now appear in iTunes.
Go to the Applications tab for the iPhone. You should see the applications you previously transferred from the other iPhone in there. Choose the ones you want and when you are ready, click "Sync" at the bottom. The applications will transfer to this iPhone, and you're done.