Barring a last-minute upset and excusing the Church of the Larger Fellowship (which seems to be something of a legal fiction in our polity), the largest membership increase reported was for All Souls Church, Unitarian, Washington, D.C.. It gained 107 members: from 401 to 505, or a 26.7% increase.
Living all of ten minutes' walk away, and at liberty from the pulpit for a spell, I had to see All Souls again. (Their website)
There was a steady flow of people before and after me, and by the time the procession begun, the main body of the church was packed and the galleries (so far as I could see) were well more than half full. Though a mixed crowd, the congregation did skew very young. The interior was the predictable plate glass and neo-Georgian architecture that typifies the New England meeting houses in exile. I did note it was a bit more spruce than the last time was there - for senior minister Rob Hardies's ordination and installation in 2001 - and didn't meet a hint of dust or dirt. The service, too, had a predictable format - New England hymn sandwich - with its conspicuous sermon and (as you woul expect for All Souls, Unitarian) a good deal of good African-American music.
While it wasn't my preference theologically or liturgically, I didn't leave wondering if I had attended a worship service at all, as has happened far too often in Unitarian Universalist congregations. It was warm, nourishing, and I can see why ASCU would grow so well and so fast. If I were church-shopping (as a person in the pews) and didn't have strong Christian commitments, I'd probably go regularly. I'll mentioned what I three things think other churches should take note-of.
1. Visitors are really welcome.. Visitors were asked to stand (and got applause, which I detest, but I know others don't mind) and first-timers were invited to stay after for lunch - a usual program I gather - for which the church would treat. Then there was the visiting/"sign of peace" moment, which made me feel reasonably welcome. Had I stayed for coffee and been a "plain newcomer" I might have sought out of of the people I had sat with to chat. So that's a good idea. The reason I didn't stay for coffee was that there was a line of people into the fellowship hall, and I gathered there just wasn't room. We should all have such problems.
2. The service was missional. I don't know if it is intentional or not, but the service is easy to pick up, and approchable to unchurched people. In other words, the action of the service matched a goal of welcoming new people. Even the sermon worked towards this: today it was about worship and the need for church itself. Directions were clear, and the order of worship helpful. Nothing was too hard to sing, except perhaps for one hymn (which is a favorite of mine so I won't complain, but perhaps I should.)
3. No Godphobia. This service included God, and God wasn't continuously constrained and explained as I've seen elsewhere. I think I mind a service that apologizes for God more than one that doesn't mention God at all. At least in a nontheistic service, your brain and backbone aren't being assaulted. And I can't help but think visitors can pick up on the prevarication. Do it, or don't.
One way this worked: the Rev. Shana Goodwin's prayer flowed from the immediate and congregational (nearly announcements), flowing to corporate personal intercession (no microphone with which to posture - thank God!), to two quasi-collects of intercession and then silence. Mindfulness (rather than petition) was her watchword, but seemed to be to be more about care-tending and carefulness than another wave of pop-Buddhism. It also passed the "Scott Wells-is-it-prayer" test: I felt led to say the Lord's Prayer afterwards as a conculsion, not a rebuttal.