Currently, there is no way to start an envelope, save it, and return to it later to finish reviewing and sending. You can kind of do this in the AssureSign workspace, which retains your current work until you send it or clear the workspace, but this prevents users from sending other envelopes while one is in progress.
I would like Nintex to add a feature where users could start and envelope and save it somewhere to pick up and finish later. This would be helpful in cases where users have over a dozen documents to compile into a single envelope when they don't have all the documents available to upload and send out at once. Users could get some documents ready for signature, then come back and resume the envelope when the rest of the documents are available to upload.