If one enters during the adhaan, the two rak'ahs should be performed immediately in order to be able to listen to the khutbah. It is reported in authentic hadeeth that a special angel takes record of those who come early to the Friday prayer. Those who show up earliest reap the greatest reward, and the reward decreases for those arriving until the adhaan. As soon as the khateeb ascends the minbar, the angel stops taking record of who enters. Speaking during the khutbah is a forbidden act and nullifies the Friday prayer for the one speaking.
Any disruption is such a despised act that even telling someone to stop speaking is forbidden.
Only the imaam is allowed to correct this. Latecomers to the Jumu'ah prayer are urged to not disturb the khateeb or the congregation. It is a common misconception that working one's way to the front row is commendable. In fact, latecomers should sit wherever there is an available space, and they are forbidden to trespass the rows. If one missed a rak'ah, the salaat should be continued to complete two rak'ahs.