In colloquial English, there really isn't much of a difference in everyday usage. We could get all technical on you and say 'met' simply means 'to meet' which can refer to the first moment of being with your friends. And, likewise, we could say that "saw" simply refers to having those friends visible to you.
But that is not how we use those phrases. They both mean the same thing...that you went out and you were in the company of your friends.