This function is part of the Multisign module.

1. Select the Transactions tab.
    
2. Open the transaction details where the signer(s) must be modified.
    
3. Click Modify processing options in the Navigation pane under Actions.
    
4. Change the Signer(s) to invite with the < > buttons and click the Submit button.
    
    
   

The signatory will receive an invitation message under My messages, or, if set via preferences, via e-mail.