The Progressive Friends were a group that hasn’t yet got their props from Quaker historians. There isn’t space here for an outline of their fascinating history, except to say you can find out more here and here.
But in sum, they started as liberal rebels in mid-1800s America, who took on a hidebound Hicksite Establishment. And they ended, invisibly but unmistakably, as the seedbed and founders of modern US liberal Quakerism. The fact that almost nobody knows this is a shame, but no surprise given the general ignorance of Quaker history among Quakers. (I’ll rant about that some other time.)