The US has been the second or third largest oil producer for decades, well ahead of all other OPEC countries except Saudi Arabia. See: http://www.eia.gov/energyexplained/index.cfm?page=oil_where

In addition to being one of the largest oil producers for decades, it has also been the largest oil consumer. Therefore it has not been in the US's interest to join OPEC. The US would prefer lower oil prices. There's no reason to expect this will change if the US moves from number 2 or 3 producer to number 1.