Groups are collections of users. They are useful for keeping track of everyone in a lab, or everyone in a particular academic department. You can browse the articles posted by the people in any group, and you can ask to join any group yourself. If you cannot find the group you are looking for, you can create it.
We'd really like you to use the search function to find groups which might be of interest to you. If you really can't find what you are looking for this way, you can browse the list of groups. There are lots of groups, so you might find it frustrating, though. Don't say you weren't warned.