Opening a new credit card could potentially lower your credit score in the short term because the credit card issuer has to perform a hard inquiry on your report.
However, having another credit card can help your score in the long run because your available credit will go up. The more credit you have, the better it is for your score.
Ultimately, it depends on your ability to manage credit. Those who spend recklessly and don't pay off balances will run into trouble, while responsible borrowers won't have anything to worry about.