You can add message templates in settings -> message templates ->recipient groups -> add a new recipient group.
Give the list a name, which will be shown in the send window’s drop-down menu. Add the signers’ email addresses and, if you choose to, mobile phone numbers. Remember to click the + button after each signer you added to include them on the list. When done click save.
When you make an invitation, you can use the recipient list, with all the signers on the list added automatically as signers.