A U.S. Citizen who wishes to marry a non U.S. Citizen or permanent resident can help their fiance obtain permanent residence in different ways.
One way is to apply for a fiance visa: If your fiance is overseas and you want to marry in the United States. This lets your fiance enter the United States for 90 days so that your marriage ceremony can take place in the United States. Once you are marry, your spouse can apply for permanent residence and you can remain in the United States while your application is processed.
When a U.S. citizen wishes to bring over a fiance, he or she will need to demonstrate that he/she has met their intended partner within the past 2 years, that both parties are free to marry, and that there are sufficient financial resources to support the future spouse.
After the I-129F is approved, the K-1 visa will be issued to the fiance and the K-2 visa can be issued to the K-1 Fiance. Then within 90 days of entering the U.S. the parties must marry and the K-1, K-2 applicants can apply for adjustment of status.
If the K-1 fiance does not marry within the 90-day period, he/she will be out of status after the 90-day period, will not be eligible to change visa status to any other designation and will be expected to leave the country.
Can my fiance work in the United States while on a fiance visa?
After admission, your fiance may immediately apply for permission to work. Any work authorization based on a non-immigrant fiance visa would be valid for only 90 days after entry. However, your fiance would also be eligible to apply for an extended work authorization at teh same time as he or she files for permanent residence.
What if my fiance is already in the U.S. in another status and we decide to marry now?
If your fiance is in the United States and entered using a visa other than a fiance visa, and you marry, then you may file an I-130 relative petition for him/her as your spouse. He/she may be able to file form I-485 along with your petition.
K-3 and K-4
When a U.S. citizen has already married a foreign spouse, then instead of the K-1, he/she can apply for the K3 visa to enter the U.S. In order to be eligible for the K-3 visa, the applicant must show a receipt from a previously filed I-30 petition, proof of the relationship and a copy of the marriage certificate.
After the I-129F is approved, the K-3 visa will be issued to the spouse and the K-4 visa can be issued to the child of the K-3 spouse, and then they will enter the U.S.