If an individual marries a US citizen, he or she may apply for permanent residence (green card) and obtain an EAD.
If the spouse is located outside of the US, the US citizen may apply for a K3 visa to allow the spouse to enter the US and obtain permanent residency. If the couple is not yet married and the foreign fiancé(e) is located outside of the US, then a K1 visa is appropriate.
If the spouse is located in the US, the US citizen may sponsor the spouse for permanent residency, as long as the spouse entered the US legally, and obtain a work permit and advance parole.