System groups has heirarchy. The topmost is superadmin, followed by admin. The default groups is User which is, considered an employee on this system and it represent the lowest level.
If you checked a group, all permission of that group will be added to the user. You don't need to add one-by-one. But If you like to specify permission, you may modify it under the User Permission
Permission can be assigned even if the group is not selected. The only problem is that, navigation menu filters group membership instead of permissions.
Click on the checkbox to select a group and click on the submit button. The system will automatically add permissions to the group.
To change user permission, go to User Permission