To use this feature the people you send messages to must also have an iPad, iPhone or iPod Touch which is on OS5 also and have iMessage set up. Your contacts are in your contacts app, so you can add more here. When in iMessage press the icon that looks like a pencil to paper, then press the + sign which will bring up your contacts list.
Your message history will be there and when you want to contact the same person just select it here.
iPad 2 3G 64gb