How do i add the other phones on my contract to Find My Phone

To expand on SJF and Chris's answers, unless you are the sole user of the phones, each one should be logged in to a different Apple ID to preserve the privacy of the contents, messages, texts, etc. Then you should link them using Family Sharing. See: Family Sharing - Apple Support

Phones linked using Family Sharing can use Find my iPhone to track any of the linked phones. In addition, you can set up Find my Friends, which is a better way to routinely find where your associates/family members are, because there is no danger of accidentally erasing any of the phones (which you can do with Find my iPhone).

Simply sign in with the same Apple ID for iCloud, and ensure that Find my iPhone is turned on:

Prior to iOS 10.3:

Settings > iCloud > Find my iPhone

iOS 10.3 and later:
Settings > (your name) Apple ID > iCloud > Find my iPhone

To be able to view other devices in Find My iPhone, they would either have to have the same iCloud ID as you, or if you are in a Family Sharing group, you would be able to see them. You cannot add other iPhones to Find My iPhone except for that. You can use Find My Friends for others, they just need to invite you to share the locations there.