I got a B1/B2 visa, can I marry with my girlfriend who has an I visa?

[Thiago Dezan]

I'm a brazilian in the US with a B1/B2 visa, my girlfriend is also brazilian and has an I visa, can we get merry in the US? If we marry can I get an extension of her visa, or can I stay with her in the US without the 6months limit of the b1/b2 visa?