Is it possible to use RAM from other computers to assist with Mathematica Light Grid Server 7 parallel computing? If so, how?
Mathematica implements parallel computing by using several kernels (several processes) that may even run on different machines. There is one main kernel which you interact with directly, and several subkernels (controlled by the main kernel) that do the work.
So, in short: you cannot have direct access to the memory of the other machines, but you can fully exploit it by running a subkernel on each machine.
You did not mention the specific problem you are trying to solve, but if you need to process a large dataset that does not fit into the memory of one machine in parallel, you can do this:
One important point: to explicitly prevent some variables from being automatically shared between kernels (and taking up valuable memory), you need to put them into a separate context, as described here and the links therein.