How can I participate in a mining pool that is mining Litecoins?
Litecoin doesn't use the same algorithm as Bitcoin. Bitcoin hashes are SHA-256 based.
Litecoin hashes are Scrypt based. You must use a miner capable of understanding scrypt.  Early in Litecoin history there were no GPU capable miners as the performance of GPU relative to CPU was very poor.  In time GPU performance increased and today most mining software supports both SHA-256 and Scrypt.  Part of the reason that GPU mining is possible is that the Scrypt parameters used in Litecoin (and other clones) have been selected to reduce the "memory hardness" of the algorithm.  The default parameters for scrypt are n=2^20, r=8, p=1.  The Scrypt used by Litecoin uses the parameters n=2^10, r=1, p=1 making it roughly 100x less memory hard and allowing GPUs to out perform CPUs while they are unable at n=2^20, r=8, p=1.