Unitarian Universalist Association Board of Trustees minutes have been published online since 2003, so it's possible -- through not convenient -- to see which churches have joined the UUA since then. Follow the resolutions.
The same sections of the minutes include other congregational news: name changes, district changes, a merger (the two churches in Queens, New York becoming one) and the disaffiliation of churches.
Many of these disaffiliations are the natural tail-end of Canadian Unitarian Council autonomy. In the other cases, the congregations left for reasons unrecorded or, more likely, died.
So before I post a list of all the new churches since 2003, let me note those non-Canadians that are no longer with us.
|unnamed in minutes||Coronado||CA|
|Panthea Pagan Fellowship||Hoffman Estates||IL|
|UU Fellowship of Johnson County||Prairie Village||KS|
|UUs of Northern Kentucky||Covington||KY|
|UU Society of the Western Reserve||Painesville||OH|
|First Universalist Church||Woonsocket||RI|
As you might guess, that loss at Woonsocket particularly hurts. (Also, because these are reported losses, I wouldn't read this as comprehensive.)