What is game centre account do I find it in the sims game map?

Game Center is a separate app in iOS. It is just one account that keeps track of all your scores in different apps, and for games like the Sims Freeplay, you can save your game using your game center account. On the first page on your device, look for an app with a white background and some different colored bubbles. Below it, it should say "Game Center". Tap on that and log in using your Apple ID and password, and it will log you in. Then just do that whenever you need to log in, and after you've logged in than it will show up as logged in on the app.
To add people, go into Game Center while logged in and at the bottom of the screen tap on the icon that says "friends" and up at the top on the left side you will see "Friends" in big black font. Tap on the little blue plus sign next to it and after another box comes up, type in the username or email of the person you want to add in the box that says "To:" and you can send them a message to let them know who you are, or you can just tap send.
They don't automatically accept though, so just give it time for people to accept your invitation.
You can find other people's usernames/emails here on SuperCheats in this topic -http://forums.supercheats.com/video-games/the-sims-freeplay/132127/Sims-freeplay-neighbors-Game.htm?page=136