When I was in London last month, I davened by the Rademishel Schtiebel in Hendon (Beis Chunuh, also known as R' Duvid's). I noticed at Kabbalat Shabbat that the tefillah of Barchu was prefaced by the word ולאמר, a usage which (if the Chabad siddur is to be believed) is from a longer passage in the Zohar.
Why does Rademishel (or any other chassidus which has this nusach) preface Barchu with velomar on Friday night?
The word ולומר being prefaced to Barekhu occurs in the context of the Hasidic practice of reciting a passage from the Zohar, כגוונא, before Ma'ariv on Leil Shabbat.
The passage in the Zohar includes the sentence ולומר ברכו את ה' המבורך. This is because the Zohar passage is describing what happens during the Leil Shabbat Ma'ariv prayer and, at that point, introduces the words of Barekhu in order to comment on them.
Reciting the Zohar passage before Ma'ariv is an act of Torah learning, while reciting Barekhu is an act of prayer (it is the official beginning of Ma'ariv). By having the prayer leader say the word ולומר immediately before Barekhu, the distinction between learning and praying is blurred, with the words able to function both as the recitation of the line from the Zohar, and as the official recitation of Barekhu.
Different versions of the Hasidic prayer tradition omit or include the word ולומר. I.e., they either end the reading from the Zohar passage right before ולומר, and proceed to the regular recitation of Barekhu (Hatzi Kaddish and then Barekhu, in the nusah of the Ba'al Ha-Tanya), or they continue reading the Zohar passage for that one additional word, ולומר, and flow into the words of Barekhu, in that double-facetted way of learning and praying simultaneously.
Nusach Sefard recites a passage from the zohar on friday night's before barchu called kegavna. The passage that we say ends with saying that with our new souls for shabbat we can bless but it actually continues and says that what we are saying is barchu et hashem hamevorach so some places end the public reading of kegavna with the word velomar and then jump into barchu so that the connection is really clear. With our new souls we can now praise god and say barchu.
