"Chat History" applies to users who opt to have the 'Live Chat Feature' in their systems. "Chat History" between employees is something only accessible to the admin.
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This can be accessed by going to "Manage > Human Resources" then selecting the employee's name in which you wish to access the chat history for.
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Once you select a name of an employee, you will see the "Identity Page" for that employee.
If you look at the tabs beside the identity section you should see where it says "Chat History" and select that tab. 

Once in chat history it will prompt you to select which employee you wish to view the conversations between. (in red below)
​(Meaning a conversation between the employee you have selected and any other employee they have chatted with) 

On the left you will see all the employees in your system to select, and to the right you will see the conversation(s) between them and the selected employee with time/date stamps. (in green below)