I would actually put it as 3 types of people who are part of ComSoc:
1. Members of FCE (Faculty of Commerce and Economics) who are automatically members by virtue of being in the faculty but do not get involved in any of the activities that ComSoc hold.
2. People who get involved in ComSoc actvities. They generally don't have to be from FCE, as long as you pay the entry fee for taking part in the event ComSoc doesn't really care which faculty you're from, although the events are tailored and aimed at FCE people. This can include:
i. Going to the Commerce Cruise, Commerce Camp or Commerce Ball.
ii. Competing in one of the competitions it runs annually such as the portfolio competition or the sports day.
iii. Taking part in the peer mentoring program.
3. Active members of ComSoc. Again, they don't have to be FCE students. They help to organise the events that ComSoc holds annually. They are the camp leaders, the lecture bashers*, the coordinators, etc. These people are usually the ones who run for positions on the executive of ComSoc at the end of the year (the exec is the group that runs ComSoc and has positions ranging from Secretary to Social exec to President to Treasurer).
ComSoc generally runs a stall during O-Week to get people to come to activities (i.e. #2) and to get volunteers for the events (i.e. #3).
There are also some competitions that are run in association to ComSoc (they are not technically part of ComSoc but are run through it). They are the PWC Economics and Business Studies Competition and the BCG Business Strategy Competition. I don't know too much about the BCG Comp, but I have been on the organising committee of the Eco Comp for the past few years and I have found it really rewarding and a great way to meet new people. I've also volunteered for a lot of other activities within ComSoc over the years including being a Camp Leader, helping out at the Cruise, being on the Executive and helping out on the ComSoc stall. I've met some of the closest friends at uni that I have met by getting involved in ComSoc.
*Lecture bashing is jargon for making an announcement in front of a lecture theatre before the lecture starts.