LogAddMember
data class LogAddMember(val convoId: String, val message: SystemMessageDataAddMember, val rev: String)
NOTE: This is under active development and should be considered unstable while this note is here. Event indicating a member was added to a group convo. The member who was added gets a logBeginConvo (to create the convo) but also a logAddMember (to show the system message as the first message the user sees).