Use the Custom field groups form to organize your custom fields. It is not mandatory to assign a custom field to a group. Custom field groups allow you to define which employees have rights to use custom fields. Link a custom field to a group, and specify which users have rights to that group.
For example, if an employee creates a custom tab, he or she must at least have View rights to be able to see custom fields in the Field chooser and to be able to add them to the custom tab.