As this post points out, it is legal for US corporations to hold an ICO and US citizens may participate. However, there are restrictions to both of these cases. The SEC just released a statement saying that they are going to investigate ICOs as potential securities more closely going forward. With respect to participating, it actually depends on the state in which you reside. For example, if you live in the state of New York you cannot participate in ICOs, as New York has stricter securities regulations (that's where the New York Stock Exchange is so they have unique financial laws).