What types of groups are there?
There is a group for everything and if not then you can create one for it. A group can be based around a location, a hobby, or any another area of common interest.
Location based: You could have a group for a school, university, neighbourhood, workplace etc. This means you are trading with people who share a common place of interest with you.
Hobby/Sport: Do you have a hobby or interest? Then why not join a group for it? This could be anything from skiing to knitting.