The Related GRNI functionality allows you to see all GRNIs for all of the related transactions accounts linked to a master account when posting an invoice.
In order for this to be available, you will need to contact support at either 0121 697 6500 or raise a chat in the widget on the right hand side.
Once this has been enabled, go to Accounts > Purchase Ledger > Master Accounts, enter an Account Code, and click Search.
Click on the Account Code and the parameter is under the Analysis heading. Click Amend which will allow you to tick the 'Related GRNI' field. Click Save when done.