How do I ask someone to be my friend?

I took an advanced guitar course recently, and I meet someone there who seemed like a cool guy. We started talking and the same night we went out for a beer. We had a great time and it turned out that we had a lot in common. We exchanged phone numbers and went our separate ways. A couple of days later he introduced me to some of his friends, and we all went out to eat. We had a private little party and they told me that they were going to make a metalcore band and that they would like me to join. I accepted the invite and the last couple of weeks have been filled with song writing, band practice, and tons of fun. I've settled with that he's an amazing guy, and I want to be his friend. But I don't know how to ask him. Can someone help me, please?