I believe that you are talking about Act II, Scene 3. In this scene, Don Pedro and Leonato are trying to trick Benedick into thinking that Beatrice is in love with him.
So, what they say is going on is that she is in love with Benedick.
it you told me of to-day? that your niece Beatrice was in love with Signior Benedick?
They say that they know this mostly because Hero has told them. They say Hero says that Beatrice is acting so much like she's in love that she can't possible be faking it.
O God, counterfeit? There was never counterfeit of
passion came so near the life of passion as she discovers(100)
They don't want to tell Benedick because they don't think he'll believe it and they don't think he'll love her back.
To what end? He would make but a sport of it and torment
the poor lady worse.