I don't believe Mr. Penn was on trial. He was one of the jurors that refused to convict.
The Indictment sets forth, ‘That William Penn, Gent. and William Mead, late of London, linen draper, with divers other persons to the jurors unknown, to the number of 300, the 14th day of August in the 22d year of the king, about eleven of the clock in the forenoon, the same day, with force and arms, &c. in the parish of St. Bennet Grace-church in Bridge-ward, London, in the street called Grace-church street, unlawfully and tumultuously did assemble and congregate themselves together, to the disturbance of the peace of the said lord the king: and the aforesaid William Penn and William Mead, together with other persons to the jurors aforesaid unknown, then and there so assembled and congregated together; the aforesaid William Penn, by agreement between him and William Mead before made, and by abetment of the aforesaid William Mead, then and there, in the open street, did take upon himself to preach and speak, and then and there did preach and speak unto the aforesaid William Mead, and other persons there, in the street aforesaid, being assembled and congregated together, by reason whereof a great concourse and tumult of people in the street aforesaid, then and there, a long time did remain and continue, in contempt of the said lord the king, and of his law, to the great disturbance of his peace; to the great terror and disturbance of many of his liege people and subjects, to the ill example of all others in the like case offenders, and against the peace of the said lord the king, his crown and dignity.’
What say you, William Penn and William Mead, are you Guilty, as you stand indicted, in manner and form, as aforesaid, or Not Guilty?
— http://www.constitution.org/trials/penn/penn-mead.htm