From Chapter 11: "While Captains Wentworth and Harville led the talk on one side of the room, and by recurring to former days, supplied anecdotes in abundance to occupy and entertain the others, it fell to Anne's lot to be placed rather apart with Captain Benwick; and a very good impulse of her nature obliged her to begin an acquaintance with him."
As it happens, Anne & CB have enough interests in common to have an interesting conversation, but what if they didn't? It would have been extremely awkward for Anne, and again she's just left to shift for herself. This is one of my favorite scenes from P2, by the way. Especially how a certain someone is watching them :P