Make sure you understand the technology and feel comfortable presenting it. If you've never done a training session on the Internet before, you may want to manage the session closely. It's hard to keep people's attention when the Internet is up in front of them. Make sure people know whether they are supposed to be listening to you or watching the computer. If you are comfortable with training people to use the Internet, you may want to have intervals in the session where people listen to you talk, then explore on their own, then come back and discuss as a group.
People learn at different rates.
It helps to know the expertise of your group to know how to gauge the pace of your training and you may want to assess their levels before the training session. If you have a very heterogeneous group, you may want to consider multiple training sessions, e.g., first training the people who will learn quickly and then recruiting from them to help you at the next training you have for people less familiar with technology. Encourage ownership in the process from the beginning. In a training, you may want to log people in as guests, show them some of the relevant features, and then have them apply for membership on the website. It helps keep their energy up and their attention.