The question of how a Christian church could join the UUA - and stay Christian over the long haul - has plagued me, given that only one new-start Christian church has joined the UUA since 1961.
The problem in part lies with the bylaws, which I reviewed last October in "Polity quandries and the UUA Bylaws". The rest of the problem lies in the question of "What is a congregation?"
It is time to put this worry to rest. First, go look at the cell/congregation/celebration diagram at Trinity Church, Dublin and come right back. Then we'll talk polity.
Trick question: If Trinity wanted to join the UUA, what would join? The most basic units, that is, the cells? (Forget the thirty-member minimum rule for a moment.) The congregations? The unified "celebration"? Trinity makes a good example of polity being distinguished by the fine points.
If you look at the cells as the basic unit, this system looks episcopalian, with the Trinity leadership as the episcopoi, which you could translate as bishops or supervisors, depending on how you saw it. No way for the UUA.
If you saw the congregation as the basic unit, and assumed some mutually-responsible spiritual leadership in the cells (as you ought) then the system looks presbyterian in the Reformed sense of having governance through classes. No way for the UUA.
If you look at its largest gathering ("celebration") as the institution that joins the the UUA, we get congregationalism again, but only in the sense that it is self-governing and autonomous, and that the UUA would be deliberately agnostic about its internal organization.
After all, what is the difference between this and the large "member society" (with special "celebrations," including a common funding source), divided into a 9am and 11am service ("the congregation") that are themselves spiritually reinforced by Covenant Groups ("cells")?
The reality is that most churches, whatever their polity, have congregational, presbyterian, and episcopalian traits, and these are held in relative tension and with varying values. A state church can reinforce these fine points by distinguishing churches and super-church structures geographically, and I wonder if the proviso in the UUA bylaws letting a local church sign-off on the creation of a new church (in essence, dividing the geographic parish) is an anachronistic vestige of New England Congregationalism being state-supported. Baptist or UCC (apart from the Hungarian ethnic Calvin Synod, which is supposed to have one) pastors who style themselves bishop, (or indeed, the Universalist minister in South Carolina who did this in 1830s) point to the fact that the New Testament endorses no system of polity, and there will be places where cause friction. Indeed, among the Seventh Day Adventists, "congregationalism" is treated as a near-heresy, and "creeping congregationalism" is as feared among the Presbyterians as "creeping presbyterianism" is feared among us.
But back to the point, and evoking the earlier church-parish distinction: let the highest-level organization (hereafter, parish) join the UUA, and let the "congregations" or cells (details to be worked on later) gather as churches under "compacts" (a Universalist use).
Let the parish call the pastor with the consent of the congregations and independent cells. (I imagine a "representative town meeting" model may have to be created.) Let the parish maintain fellowship between the elders and deacons that the congregations and independent cells - for these are Christian churches in the proper sense - raise up in a fashion not unlike the way the Ministerial Fellowship Committee (in a system carried over from the Universalists) maintains fellowship between ordained ministers. But more, and like the older Universalist model, let the parish fellowship committee set the standards of fellowship for the congregations and independent cells themselves.
It may not look like congregationalism like most UUs know it, but it work with the limitations established by the UUA and is faithful to the Universalist tradition. More than this, and in the absence of a catholic and ecumenical polity model, it should work, and help grow a newly started church, er, parish.