1. A method of hosting electronic messages, comprising: at a server 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 participants of the conversation, including: receiving content from each of a plurality of participants of the conversation and transmitting the received content to the other participants of the identified set of participants; and upon an additional participant being added to the conversation, providing content of the conversation to the additional participant that includes all content provided to the identified set of participants, and adding the additional participant to the identified set of participants.