chat
Web client
We use Quakenet IRC as our chat provider (channel #bots4). You can connect using this web client without downloading anything. It may be helpful to open this page in a new window so you can play and chat at the same time.
Don't be surprised if no one is talking. Many of us idle in the channel while away from our computers, so it might take some time before someone responds if no one is around.
mIRC
If you're a regular player, I strongly recommend downloading a standalone client to run on your computer. They're more customizable and easier to use once installed. Many people run these clients in the background and remain connected even while idle, not only while playing. I recommend mIRC. It's popular, free, and easy to setup. If you get stuck installing or configuring it, ask for help via the web client or on the forum.
Other clients
There are other IRC clients available besides mIRC. If you don't want to use mIRC or can't get it to work, you could start by looking through Wikipedia's comparison of IRC clients to find one that suits your needs.