Hanging out with friends?

I have know these girls for over 2 years and I consider us friends. But I never hang out with them outside of school. They sometimes hang out with each other and I am never included I am about to be a senior and I want to have friends that will last for a long time but I get nervous when I want to hang out with them and never ask they think it is stupid that I get nervous but I don't want to look stupid I feel if we hang out our friendship will get better and we can be good friends and that is what I want but how do I do that