Edit IMArchiveDisplay.jsp.
Search for the following the code lines in IMArchiveDisplay.jsp:
.... ht.put("has_joined_the_room","<span class='user'> {0} </span> <span class='headervalue'> has joined the room.</span>"); .... |
Replace the headervalue with the desired text.
For example:
.... ht.put("has_joined_the_room","<span class='user'> {0} </span> <span class='headervalue'> has entered the room.</span>"); .... |