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