How can multiple users edit the same submission to the webform ?

For example, user1 completes first 2 pages of the survey webform and saves it. user2 should be able to access this saved submission, modify first 2 pages (if necessary), and complete another part of the survey and save. user3 can now access, and so forth.

The only way you could do this would be by having a single user account that all the users signed into. Then you could use the normal save as draft ability and Webform would pre-fill the forms up to where the last person left off. However, it won't start the user at the left-off location, it'll start over again.

Of course you probably won't want users needing to share a username/password, so you'd also need to come up with some way to log users in before they fill out the form, something like a one-time link that logs them into the system with a generated username. If you wanted to build this all out with custom code, it'd be likely you could have a single button that both logged a user in, and took the user to the correct location in the form based on that user account, though you'd need a way to distinguish between different groups of users. There are a bunch of unknowns here, what happens when two users try to fill out the same form at the same time? How do you separate different submissions? What happens if a user fills out 1 page and then closes the window, without explicitly saving?

The use-case here isn't common enough implement in the module directly, but it could make for a fairly interesting contributed module.

Answered in #2