The MGM signature has their own pools that are for guests only. It had plenty of lawn chairs during the day on Memorial Day weekend, and music that gives a more Vegas party feel. You also get access to MGM's main pools.
They let us use them before we could check in at 3pm, and didn't fuss about us bringing our own drinks into the pool area.
The MGM grand and MGM signature are connected via a short enclosed walk.
Our host answered all our questions quickly and thoroughly. I would stay here again!