How about a combined setup?
Use a Jabber server that supports LDAP authentication (many do; we use ejabberd at work, which – together with its mod_shared_roster_ldap – can even pre-fill the roster with groups (such as, employees grouped by location) from the LDAP, and with mod_vcard_ldap which makes personal info such as first and last names, etc. available in the Jabber contact information).
Then, install Jappix on your webserver and configure it to connect to your local Jabber server. Users can then talk to each other (by text chat, or even audio or video chat).
Your users would even be able to skip the web thing and use their favourite Jabber client, so they would not need to be restricted to access it only on your webserver. They will love it (at least those who are a bit more technically inclined). Also, it’s a standard and open protocol.
For chat protocols/logging, I suggest a server-side solution (which will work unless someone uses something like OTR, E2E or PGP – but these are for one-on-one chats only, not the “MUC” multi-user chatrooms). Jappix also has a “save” button in which a user can opt to save their chat log.
If you want to permit talk only between people who actually have accounts on your server, you can disable the “federation” feature of Jabber, usually called “s2s” (server to server communication). We do that on our company Jabber server as well, since it’s for internal communication only.
Actually… this question gave me the idea of installing Jappix at work too ;-)