If you're looking to start a new conversation with her, you can hover over the inbox, click Start New Conversation, and then type her name in the "Participants" section. It will drop down from a list, and you just click that name. Alternatively, you can just go to her page and click "Start Conversation". Hope that's what you mean. ;)
If you're looking to add her to a current conversation, you have to be the 'host' of that conversation. If you want to add her that way, and you're the one who started the conversation, you can just go down a bit on the conversation's page, and click "Invite More" where is says "Conversation Participants"
If you want to add her to a conversation that is already started, you can look on the left hand side of the screen in the message's page and push the Invite More button. http://puu.sh/bpV3p/b68bb8813e.png
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