If she is my fiance and we are not yet married, can she still come to the US?

If you are a U.S. citizen, you may apply for your fiancee to get a K-1 fiance/fiancee visa that would authorize your fiancee to enter the U.S. and then become married to you within the required time; after becoming married you could apply for her to adjust status in the Immediate Relative visa category to become a Lawful Permanent Resident (to get a "Green Card"). It would not be permissible for your fiancee to come to the U.S. on a B1/B2 visitor's visa or through the Visa Waiver Program if it was her intention to become married and remain here permanently. That same analysis would not apply, however, if you and she genuinely have not decided whether to marry or if you and she plan to marry but genuinely have not decided to live together in the U.S. If a couple were to believe that analysis does not apply to them, then there are additional considerations to take into account at the earliest possible time regarding documentation and timing. Whether with a K-1 visa or otherwise, it would be wise to engage an immigration attorney who, after learning all of the relevant information about you, your fiancee and your goals, could advise you about immigration eligibilities, options and strategies and could offer legal representation in the application process that often is significantly more complex than it may appear.