Back in the 1800's, a group of men in Plymouth, England met together and expressed dismay at the state of the English church. They broke off and formed a group, which tried to be scruplously non-denominational.
Their initial major teachings were what they called "New Testament church principles," which included a radical departure from the clerical system throughout Christendom, choosing instead a structure based on several lay elders. The founders included J.N. Darby, a lawyer-turned-teacher.
Darby came up with the idea of dispensationalism, which is the basis of the modern premillenial view.
Of course, I skipped over several schisms which blot their history. To be honest, I feel little loyalty to the group. I am a Christian first, a Calvinist second, and Plymouth Brethren a distant third.
The branch I am affiliated with, the Open Brethren, is little different theologically than many independant Baptist groups -- fairly fundamentalist, openly dispensationalist, and so forth.
I'll support the Assemblies (thats the other term we use to describe ourselves) as long as I remain convinced they promote sound doctrine and operate Biblically.
I came close to leaving when, in a Youth Conference setting (which is designed to reach out to unbelievers, carnal believers, and new believers), the keynote speaker saw fit (using shoddy exegesis) to attempt to disprove Definate atonement to an extremely impressionable group. (I jave no problem with discussion in the proper forum: that one was not).
Even if I become amillenial (and I will, if my studies lead me that way), I will stay there as long as they allow me, anda will not be divisive about it. Thats not a dealbreaker.
If I were forced to choose between my association with the Campus Crusade for Christ and the Assemblies (and that is not likely to happen, but not too far-fetched, either), I'd choose Crusade. (There is a sizable group in the Assemblies that consider Crusade too ecumenical).