Other than saving the chat log and participant's list and manually perusing it for participation, I don't know of any tools built in.
You could use a tool such as classdojo. You will need to think about if your students will find that to be a fin motivator or a silly one. The nice thing is that the interface makes it really easy on the teacher to track the student's participation. If you wanted to, you could merely use it for your own tracking if your students were not likley to be motivated by the students side of the software. The link is www.classdojo.com. Classroom 2.0Live! will have a session with the makers of the software in a few weeks.