I think Bumblebee was already on Earth when the movie started. Everyone else came later. And I think Bumblebee is the only 'Bot in this one? That may not be confirmed though.
Edit: I saw TLK like a month ago and completely forgot they retconned the Transformers to have been here all along. Wtf
In the first movie Megatron has been stuck in the ice for centuries or millenia and apart from Jazz, Ironhide, Ratchet, and Optimus they don't go into how long the others have been around. In Revenge of the Fallen they'd been on Earth in the distant past and left, but Seekers like Jetfire had been on Earth at least 100 years (one of them was a Model T). In Dark of the Moon they revealed that Soundwave has been working with human collaborators since at least the 1960s.
Basically the concept of them always being here has always been here, but with each movie they reveal more and more random Transformers stuff from the past.