You can add additional signers to a signature request at any time before the due date has passed. Similarly, you can also remove signers from a request if someone has been added by mistake.
The list of the signers invited to sign a document is always accessible from the signature details accessed by going to the Signatures tab > Pending requests and clicking anywhere on the request in question.
The new signer will receive an email notification and can immediately sign the document.
Note that a removed signer cannot be added back to this request again later. If you remove a signer by mistake in which need to sign the document, you will have to cancel the request and create a new one from scratch.