In the movie "The Chronicles of Narnia : Prince Caspian" there is a kingdom of humans called Telmarines. We know that these people are in fact humans because the White Witch says that she needs the 'blood of Adam'.
How did these humans get to Narnia in the first place? My theory is that Aslan let them into Narnia and that is why when the go through the tree portal they go home. Does the book do a better job of explaining this? Is my theory at all correct?
This is described in the C.S.Lewis novel "Prince Caspian". In short, the relatives of the Telmar were Pacific pirates who fell through a portal housed in a cave:
You, Sir Caspian,” said Aslan, “might have known that you could be no true King of Narnia unless, like the Kings of old, you were a son of Adam and came from the world of Adam’s sons. And so you are. Many years ago in that world, in a deep sea of that world which is called the South Sea, a shipload of pirates was driven by storm on an island. And there they did as pirates would: killed the natives and took the native women for wives, and made palm wine, and drank and were drunk, and lay in the shade of the palm trees, and woke up and quarreled, and sometimes killed one another. And in one of these frays six were put to flight by the rest and fled with their women into the center of the island and up a mountain, and went, as they thought, into a cave to hide. But it was one of the magical places of that world, one of the chinks or chasms between that world and this. There were many chinks or chasms between worlds in old times, but they have grown rarer. This was one of the last: I do not say the last. And so they fell, or rose, or blundered, or dropped right through, and found themselves in this world, in the Land of Telmar which was then unpeopled. But why it was unpeopled is a long story: I will not tell it now. And in Telmar their descendants lived and became a fierce and proud people; and after many generations there was a famine in Telmar and they invaded Narnia, which was then in some disorder (but that also would be a long story), and conquered it and ruled it. Do you mark all this well, King Caspian?”.
There's no special reason to assume the Telmarines in the film came from somewhere different.