Boundaries are not the same as they are in the real world. The inside of your head is no longer closed off from the world, it contains the world. Rather like that hippy chap in So Long and Thanks for All the Fish by Douglas Adams . He built his house so that the inside looked like the outside. He could retire to outside the world, and have the whole of the world in his living room.
Dreams are like that.
People turn up in my dreams all the time. You can interpret this how you will. My subconscious ploughing over issues that are of concern to me, the elastic nature of the Dreaming, Gaiman's Lord Morpheus sending people to me when I need to deal with them. People using their own Dreaming to get at mine. It doesn't make much difference.