To go IC, you simply type /ic [your character's name here]. (I'm pretty sure. I haven't been on for a while so I'm not exactly sure. Someone else can probably make sure I'm correct.)
If there's a roleplay going on and most everyone is already in character, you should probably just ask what's going on. Someone's bound to give you a quick explanation of what's happened thus far. If no one answers you, then pick a random person (preferably someone IC) and send then an in chat message asking for a quick summary. If you're interested in joining, just ask. It tends to be pretty open so that anyone can join, but sometimes groups of people will go to another room so that they can do a private roleplay over the chat.
Hope that helped. The chat is really fun but it can be pretty confusing. I'd go on more often but my internet has a tendency to cut out in the middle of everything so that I end up having to spend more time logging in than actually chatting with everyone.
FRIENDS: Will look at you like you're crazy when you tell them you're an alien from outerspace.
BEST FRIENDS: Will break you out of the loony bin and drive you to New Mexico to meet up with the mothership.
There are very few personal problems that cannot be solved through a suitable application of high explosives.
Who was the first person to say "See that chicken over there... I'm gonna eat the first thing that comes out of its butt"?