If you want to leave a conversation,  you are able to do this at anytime in "Conversation Details." Should you change your mind, the conversation administrator or a moderator can add you back. (But you can also remove them from the conversation)! 

Your messages will remain in the conversation, until the administrator has completely deleted the conversation.  Your affixed attachments can be left in a conversation or individually removed.  At anytime, but then it is final! (Gone is gone. This avoids any manipulation)!

If you are no longer registrated with Grappt, you are then completely gone. All messages and files are then no longer visible.