If you are a U.S. Citizen who is engaged to a foreigner, you can file a K-1 fiancée petition with the United States' Customs and Immigration Services (USCIS) in order to bring your fiancée into the United States legally. This is true even if he or she does not have a separate reason to be in the country. In the petition, you, as a U.S. citizen, must document that you are in fact a U.S. citizen; that you and your fiancée have met in person within the prior two years; that both of you are legally free to marry; and that you will marry within ninety days of the foreign fiancée's entry into the U.S. The regulations regarding the K-1 fiancée petition, i.e. the need to prove when you last saw your fiancée in person and when the wedding will take place, are in place to help dissuade marriages that take place strictly for the purpose of providing a foreign national with a valid United States visa.
The documentation required to prove these facts may vary, but typically, it involves a great deal of paperwork, so you should speak to a lawyer with experience in immigration and fiancée visas in order to determine exactly what is required. The process of any visa application can be months long, and sending in an incomplete application with missing documents can cause major delays or even require that you begin the process from the beginning.
Upon approval of the fiancée petition, your foreign fiancée will be required to go to the U.S. embassy/consulate in his/her home country and apply for a K-1 visa. Upon issuance, the fiancée will enter the U.S. and receive an automatic employment authorization for a period of 90 days. After the couple gets married, the alien should file the I-485 application for permanent residence.
Essentially, a fiancée visa is a way of getting your fiancée into the U.S. legelly, upon which time he or she is allowed to establish themselves by getting a job, as well as getting married. At that point, he or she must file for permanent residence because the fiancée visa is considered a temporary arrangement, only designed to provide protection for the foreign fiancée until the wedding has taken place and for a short time afterward.
Consider speaking with an experienced attorney who can provide you with guidance based on the specifics of your situation.