As was the case with almost all current West African coastal countries (and some of Central Africa), the first people in the United States from present-day Guinea-Bissau were imported as slaves.
So, since the late 15th century and with the cooperation of some local tribes, the Portuguese not only entered into the slave trade, but also imported large numbers of Senegambians (primarily of Bissau and Cacheu) and other Africans to the Western Hemisphere via Cape Verde.
The Portuguese, after buying slaves from African kings and aristocracies, sold them to the European merchants (Spanish, English, French, Dutch, Swedish).
[1] There has been a growing acknowledgment of descendants affirming their Bissau-Guinean ancestry and practices from the original traditions of African tribes such as; Balanta, Fulani, and Brame.
Many Balanta slaves imported to modern day United States from Guinea-Bissau we're sold as war captives from confrontation with the Portuguese and Bijago people.