LAN Chat Enterprise - description from Softdepia.com:
The program LAN Chat Enterprise is designed to support exchanging messages in the regime of real time in a small office, in an educational institution, in general in any office with local networks. This unique software affords an opportunity for users of the local networks to communicate easily. The program works on the conference principle. LAN Chat Enterprise requires neither connecting to Internet nor a specialized (a selected) server.
LAN Chat Enterprise is a "Free to try" program. You can download and try it for an evaluation period.