It is an English upper middle class intensely evangelical sort of neo-Anglican group. I was unaware they were any congregations in the United States.
Their denominational history began in Ireland in the early 19th century. John Nelson Darby was one of the original elders. Many evangelical denominations get their “pretrib” doctrine from the influence of Plymouth Brethren which held sway in evangelicalism of the 19th and early 20th centuries. George Muller was also one of the founders of the Plymouth Brethren movement.
Plymouth Brethren have been all over the USA for a long time. They are not a large group, but they fly under the radar because their assemblies(Chapels) are not usually formal in organization. Normally there is no “pastor”(they reject the notion of clergy) but rather the elders take turns in preaching duties. They derive their their doctrine and organization from Scripture alone - claiming they are spiritual descendants of the Apostles.