Hello,
I can see this has been asked and asked going back to 2015 at least.
I'm a project/programme manager I would like to be able to manage tasks/resource on projects for multiple clients without having to email and call every person for an update.  I have been searching for a product with the right feature set.  I had initially thought this would work and was very excited to start using it as I have mostly used MS Project.  This product seemed to have the strengths of MSP but with a less complicated and more open approach to collaboration. Boy did I run full tilt giddy and excited into the single task owner brick wall. I'm currently using the free trial and boy am I glad I did.
Unless someone "preferably"  at Smartsheet can tell me this is about to be addressed then unfortunately I will have to stop using this product as it falls short right at the finish line.  Would be an awesome product otherwise.  Its a real shame to drop the ball on such a way.  Considering how long this feature has been asked for, indefensible too.