1. A method of hosting electronic conversations, comprising: at a computer system having one or more processors and memory storing programs for execution by the one or more processors, hosting a plurality of conversations, each having an identified set of participants, the hosting including, for a respective conversation: providing content of the conversation to the identified set of participants of the conversation, including: receiving content representing a portion of the conversation from one or more participants of the identified set of participants of the conversation; and transmitting the received content to the other participants of the identified set of participants; while a first participant of the identified set of participants is providing or editing a portion of the content of the conversation: enabling a second participant of the identified set of participants to edit the portion of the content; and after the conversation between the first participant and the second participant has begun for a time period: adding an additional participant to the conversation by: providing content of the conversation to the additional participant that includes all content provided to the identified set of participants; adding the additional participant to the identified set of participants; and enabling the additional participant to edit the content of the conversation, including content added to the conversation before the additional participant was added to the identified set of participants; wherein the plurality of conversations including the conversation are instant messaging conversations, and participants in the conversation are instant messaging participants.