That depends on which version of the product you are purchasing.
The Sleep Guardian works if you cosleep.
Try to place the Sleep Guardian under the mattress where your child usually sleeps (your best guess is ok too!). Be aware that everyone on the mattress will feel the vibrations. If you cosleep with your child and another person, both may stir.
The Sleep Guardian 2, however, will not work if you cosleep. This product uses a very sensitive sensor to detect movements of the child sleeping on the bed. If a parent is also on the bed, it will detect their movements as well and will throw of the timing of the product. Therefore, it will only work if the child is the only sleeper in the bed.