Hey there! First time writing here 
Use case:
So we have this setup of internal and client asana where in the internal are all team members working on dev, design etc and the client asana is where we communicate with our clients (if that’s not obvious).
So I was assigning tasks internally left and right but a few times in a row I accidentally assigned an internal task to a client because the names match almost fully. I quickly removed the assignee, collaborator, history and all, but it’s really stressful and easy to miss.
I don’t know how many have this workflow, but I believe it’s not too rare. And having ALL members from ALL asana projects show up is indeed a recipe for disaster. You can easily see how a bad comment read by a client can make things go bad because the names match and the avatars are simple. People make mistakes
Proposed solution
All I can think of is the ability to limit what people show up in the @ mentions, Assignee dropdown etc.
When you WANT to mention someone not in a project, Asana will ask you to confirm if this is what you want. It will do it every time you mention/assign or whatever. It will also pop an option to add this person to the project or have a checkbox “don’t ask me again for X on project Y”.
This alone will remove the stress. It adds an additional step, but to me it makes perfect sense. Not in the project? Most likely there is a reason. You add it? Most likely this is a mistake, but if not - sure go ahead, just letting you know!
Thank you all for your time!