Remember that this scene is a key one as John Proctor, Francis Nurse and Giles Corey go to the court to try and argue against the madness that is sweeping through Salem. One of the many bits of evidence they try to present to the unyielding Danforth is the testimony of 91 good people who act as witness to the good characters of Martha Corey, Rebecca Nurse and Elizabeth Proctor. However, instead of having the effect they intended, the three husbands are aghast to find that by asking their friends to sign this testament they have been incriminated. Notice how Parris says the moment that the testimony is presented "These people should be summoned... For questioning." This of course shocks Francis Nurse who gave his personal word that no harm would come to those that signed.
Of course, unfortunately, this fits in with Danforth's logic, for if they are covenanted Christians, then they will have "nothing to fear." Danforth thus orders their arrest:
Mr. Cheever, have warrants drawn for all of these - arrest for examination.
Thus by trying to help their friends those that signed are swept up in the Salem witch trials too.