Groups allow you to organize people around a common interest. You can use groups to track attendance (manually or via the check-in system), schedule volunteers, communicate with members, share documents, and more. Groups are broken down into four main built-in types, but you can define your own types as well:
Volunteer - contains people who volunteer in a ministry
Small Group - contains a small number of people who meet periodically to study the bible.
Class - contains people who are going through an educational curriculum either at the church or in school. Use this type for grade levels (e.g. preschool, grade 1, grade 2).
Church - generic grouping which doesn't fit into any of the other types such as baptism groups, launch team members, and more.
Each group has privacy settings which allows control over visibility and how new members can be added. You can assign one or more leaders to help manage their group.
Groups a person is a current or past member of will appear in the involvement section of their profile for easy access. This way you can quickly see how they are involved in the church. You can even define custom fields on a per group basis so you can capture any data you need specific to that group.
Now that you have a high level view of what groups are and some of the ways they can be used, try to create a new group and start adding members.
See this article if you are interested in tracking attendance for a group using the check-in system. In this case, you would add a session and select which group rosters to use when checking a person in.