What she is talking about here is an episode from quite late in the book. She is talking about a feeling she had while spending time with Peter Wessel.
In this episode (page 218 in one copy of the book, at least), she has been sitting with him on a couch. They sort of accidentally start to kiss and she is overcome -- they come to be "clasped" and "in a whirl." So she is having a sort of a young love moment. She will later realize Peter is not for her, but for now, she feels a strong emotion. That is what she is talking about when she says they have found each other.
On Friday, April 28, 1944, Anne writes about spending time with Peter "on the divan, as usual, in each other's arms" when she experiences a sudden wave of emotion marking the appearance of what she refers to as "the second Anne"--the gentle girl who only wants to love.
When Anne stands up and goes to the window to say goodbye to Peter, they embrace, and Anne kisses him on his left cheek. The two then share their first real kiss on the lips, which sends them both into a "daze."
Reflecting upon this moment, Anne writes of the transformation within herself that Peter has witnessed. She claims that Peter needs tenderness, and that this is the first time in his life that he's given himself to another person.
When Anne remarks, "Now we've found each other," she is referring to this special moment of discovery between two young lovers. Although she also wonders if it is "right" for her to give herself over to passion so quickly, she decides that the comfort she receives in Peter's arms is a good enough reason to pursue this secret relationship.