US citizens who are engaged or who get married to foreign nationals can bring their fiancé(e)s or their spouses to the United Stated and help them to obtain lawful status. Hence, a US citizen who looks forward to bring his fiancé(e) to the United States, must file a petition for a fiancé(e) visa for his/her fiancé(e) and this visa will help the fiancé(e)s of US citizens to get into the country legally. If you wish to bring your fiancé(e), to America, you must file Form I-129F, Petition for Alien Fiancé(e) and your fiancé(e) can work legally in America, while in the country on a fiancé(e) visa after getting a work permit.
At the time of filing a petition for a fiancé(e) visa, you must establish that you are a US citizen, by providing the required documents and that you will get married to your fiancé(e) within 90 days. After reviewing your petition, if the USCIS finds that your fiancé(e) is eligible to immigrate to America, your fiancé(e) will be issued a visa and with that visa, your fiancé(e) can get into the United States but you must get married as soon as your fiancé(e) gets into the country as this visa is valid only for 90 days. After getting married, your fiancé(e) can adjust his/her status and become a Green Card holder. You need to remember that your fiancé(e) can file Form I-485 for adjustment of status only after getting married to you.
After getting into the United States on a fiancé(e) visa, your fiancé(e) can apply for a work permit by filing Form I-765, Application for Employment Authorization. But that wok permit will be valid only for 90 days as it is based on the non-immigrant fiancé(e) visa. After getting married, your fiancé(e) can apply for another work permit at the time of filing a petition for adjustment of status. In this case, your fiancé(e) can apply for adjustment of status and for an employment authorization document at the same time and file Form I-485 and Form I-765 together.