About 100 police officers were brought in to control a crowd of disappointed fans who were unable to get into the Grateful Dead’s concert at the Irvine Amphitheater. Most of the crowd outside the amphitheater dispersed shortly after a dispersal order was issued from a police helicopter. The remainder left after the officers--who were called in by Irvine police from several surrounding communities--began walking in formation toward the entrance, a police spokesman said. There were about a dozen arrests. Officers at the scene reported that some members of the crowd threw rocks and bottles. The incident occurred about an hour after the concert began before about 15,000 people.