When member sign up to your platform, also known as buyers. They will be put into a default group. You can set this default group by clicking the little yellow wheel icon next to the group row.
This is very useful for keeping all of your members segregated into smaller groups that you can contact and give relevant information to when need be by using the email members function.
To change a users group go to manage members click the little yellow wheel icon next to that users real scroll down and click on change user group.