I'm not aware of any CPU usage throttle I'm afraid. It would be a useful feature for noise/heat/power reduction, and would probably increase the lifetime of your fan bearings.
As for joining a team, login to the website, and select the Preferences option at the top left, then the required links will be under the Team Affiliation section. I don't believe the Team section in the client config file does anything anymore - I'm sure someone will correct me if I'm wrong...