You can fit up to 3 people in a Rickshaw. Log in here and invite team mates by sending them an email invitation from the Team Mates section:
Invited team mates will get an email with a link and some instructions. It's pretty simple really.
Doing this is important for a bunch of reasons, one of which being the Excellent Cool Earth charity swag sent to each team. If your teammate isn't registered, they won't get any swag.