Full Idea
This being-with-one-another dissolves one's own Dasein completely into the kind of being of 'the others', in such a way, indeed, that the others, as distinguishable and explicit, vanish more and more.
Gist of Idea
In company with others one's Dasein dissolves, and even the others themselves dissolve
Source
Martin Heidegger (Being and Time [1927], p.164), quoted by Mark Wrathall - Heidegger: how to read 5
Book Reference
Wrathall,Mark: 'Heidegger: how to read' [Granta 2005], p.47
A Reaction
He seems to be describing the psychology of someone who joins a small crowd which gradually increases in size. I take this relation to others to be the basic existential dilemma, of retaining individual authenticity within a community.