How were people able to get passage to the colonies in return for a promise to work?
This was called an indentured servant. The person promised to work as a servant for seven years in return for the cost of passage.
This was a form of slavery modeled after the Old Testament Mosaic pattern. The person basically was a slave in bondage for those seven years and then set free at the end of seven years. Also at the end of seven years the previously indentured servant was given a parcel of land.
The process was unfair as the cost of passage was much less than the value of seven years of unpaid labor. Many of the indentured servants did not serve the required seven years. Seeing the unfairness of their servitude and the vast expansion of the American wilderness, they simply ran away and started new lives for themselves and blended in with the general population.