The famous gulling scenes of this play when Benedick and Beatrice are convinced that the other is in love with them and then are themselves made to fall in love with the other, are key parts of this excellent comedy. We see that in Act II scene iii, the gulling of Benedick involves Claudio, Don Pedro and Leonato. Straight after this scene, the shorter Act III scene i, the gulling of Beatrice, involves Hero and Ursula working together to trick Beatrice.
However, when we think about their reasons for being involved, we need to go back towards the beginning of the play. In Act II scene i we see that it is Don Pedro's idea to try and convince the two thorny and prickly characters to fall in love with each other. Note what he says:
I will in the interim underake one of Hercules' labours, which is to bring Signor Benedick and the Lady Beatrice into a mountain of affection th'one with th'other. I would fain have it a match, and i doubt not but to fashion it, if you three will but minister such assistance as I shall give you direction.
Thus we can see that the characters are involved in the gulling scenes because of Don Pedro's personal plea for assistance in accomplishing, as he puts it, one of the labours of Hercules.