Groups can be used to create a namespace where you put your projects. You can also give people permissions on the group level. When you create a new project, all the group members will automatically have access to that project.
To create a new group, after we login we expend Groups and click Your groups > New Group (on the top right of the screen). Another window will be displayed to fill up group information details. (Img below).
More information about groups you can find on official documentation of GitLab at: https://docs.gitlab.com/ee/user/group/