哪种签证，可以到美国?

I have a question from one of my colleague in BJ, her boyfriend is in
American with green card. They hope to marry nowadays.
“但是据说如果在美国之外结婚，不能以家属的身份到美国生活，必须要拿学生签证，工作签证或者商务签证。如果除了这几种签证，还能什么办法去美国？“
Maybe you can have the answer. If any, please tell me.