You could always set up the Java Connect client on the server, which is a browser-based client:
Title: Deploying Sametime 7.0 Connect for browsers on a Sametime 7.5.x or 8.0.x server
Doc #: 1243158
However, if you intend to chat with anyone on Sametime, you DO need a valid user name & password to be able to log into the Sametime server with a chat client. We do not have functionality whereby you could log into a chat client anonymously and chat with other logged in members.