Deputy District Attorney Philip Daley says the charge was dropped June 15 because of insufficient evidence but declined to elaborate.
Grant, also known as "Bigg Marv," appeared on MC Hammer's 2009 reality show, Hammertime, which followed the rapper and his family at their Tracy home on the A&E cable network.
Grant says he met the woman who lives in Southern California through Twitter and the two exchanged e-mails and phone calls before meeting in person.
