Answer
Yes. New participants may be invited into an existing conversation at any time. The must be invited to join by an existing participant, they can not request to join the conversation themselves. When a new user joins a conversation they will not be able to see what has previously been said in that conversation.
GMS Instant Messenger users have the option to receive a transcript of the each conversation via email when a conversation ends so that they have a record of what was said. Users who are present for only part of a conversation will only receive a transcript of the section of the conversation for which they were active participants.
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Keywords:IM invite add new user conversation