Material groups are independent parameters and need not be assign to any company code or plant. Which ever company code or plant is being used will be considered. Material groups appear on the Sales:Sales Org. 2 view of the material master.
New material groups are created in SAP configuration. They are created as transported objects in your DEV system and then transported to the other systems in your landscape.
You can find the Configuration area using the following menu path:
Menu Path: SPRO–> Logistics – General –> Material Master –> Settings for Key Fields –>Data Relevant to Sales and Distribution –> Define Material Groups
Select the desired activity: Material group 1, 2, 3, 4 or 5
Related article: How to use reserved or additional fields in SAP.