In the background you have the returning Autobots from the first movie, Ironhide, Bumblebee, Optimus Prime and Ratchet. Then beneath then in vehicle mode are the various cars that have been spotted on Transformers: Revenge of the Fallen sets over the last few months plus a new one. From left to right, the Chevy Beat (Skids), Chevy Trax (unknown), Corvette Centennial (Sideswipe), three RetroSBK bikes (Arcee) and no clue what the purple car behind the bikes are.
Arcee's alt mode being comprised of three separate components have yet to be proven but the three bikers have been spotted on TF sets several times and now being used in a cover that is made up entirely of Autobots seems to be further confirmation of her alt mode. As for that purple car, not a clue. It could be a character created specifically for the comic book or a car that wasn't spotted on the sets but is in the movie. As usual time will tell.
Transformers: Alliance mini-series issue 1 comes out later this month and issue 4 is out in March. Check your local comic book stores about getting the mini-series.