To send a document to multiple signers, you will have to add them in an intermediate step in the sending process.
To add signers, click on the + button to the right of the name of the Signer. Don't forget to add the appropriate signature boxes (and other relevant fields) for each signer that you add.
To remove signers, click on the X within the colored box of the “Signer Y” you want removed. Remember to remove any field that you added to the document related to the signer that you have removed.
NOTE: You can add a maximum of 40 signers per document.