I already have the project I did in my final semester ready that I wanted to sell. Along with chatting with multiple persons you can also do Voice Chat, File Transfer and even know who all are online. Since we use SIP for setting up communication, integrating with some other servers also will not be difficult. But I had used an Open Source SIP server (Resiprocate). If you are okay with it, I can give you the project right away, with some minor customizations. Otherwise if I have to write a Server in Java, I can do it within 15 days. But I'd advice you to go with the Resiprocate because it's a very efficient and stable server and has been tried and tested from years. I want to stress again that it's just my frank opinion and does NOT mean that I can't write my own server.