If you read the UUA Directory, or the certified congregations roster, you may get the impression that there are a couple of dozen tiny churches, mainly in New England, that couldn't possibly have an active ministry. I used to think they were a coffee klatch-sized mob of octogenarians subsisting on Church Bean Dinners.
But this impression is wrong; many are in federated, community, and multidenominational churches.
First, a terminology check. A community church, a multidenominational (or interdenominational) church, and a federated church are not the same thing.
A community church, before the term became hot and lost any real meaning, was a solution to having too many congregations in a thinly populated area. Better to amagamate than die. Keep links, at least nominal links, to the two or more denominations, but really identify with the locale more than a tradition.
Interdenominational churches are more likely a product of ecumenical experimentation and a irenic hope. I tend to think of them as post-WWII optimism, and that's why they usually live in the suburbs.
A federated church is much odder. Take two churches, each with its own membership roster, leadership, and history. Wed them with a third board. House them in one building (or one building at a time) and let them worship together. That's federation, and it seems like a lot of trouble.
Federations can loosen, and the constitutent denominational churches re-establish independent existences, but I can't think that's very common. (But do a Google search if you're interested. I've seen news reports of it happening.) I imagine the willingness to have a federation rather than community church often has something to do with a big pile of money. But that's just a guess.
While there are congregations in the UUA in community, multidenominational, and federated situations with the American Baptist Churches, Christian Church (Disciples of Christ), the National Association of Congregational Christian Churches, and the United Methodist Church, most alliances are with the United Church of Christ.
So, a federated church will submit the membership numbers of the old Unitarian or Universalist church within the federation, and in almost every case it is far smaller than the other partner or partners in the federation.
Now, I think this may have to do something with confused figures and may give an impression that the UUA has grown less than it has; that is, some of last year's numbers were over-reported (and not that this year's are under-reported.)
As it happens, I'm moving my membership to a community church, dually affiliated with the UUA and UCC: The Eliot Church of South Natick, Massachusetts.
Link: Federated Church of Sycamore, Illinois, a Congregationalist-Universalist merger, both in the UUA and UCC.