Did you know that guests in Asana don’t always see each other, but instead they might see “Private User” on tasks?
This is usually something you learn the hard way, with clients or partners being all confused in the comments saying “Who’s Private User?”.
This is actually expected behaviour: in many cases (covered in my “Who sees whom” post on the forum), guests don’t have the permission to see each other, unless they are added at the project or team level. But we don’t always want or can do this.
So what’s the solution then? You are lucky we are really smart at iDO and came up with a solution 
→ just create a dummy project and put those guests as project members!
It won’t give them access to anything (this is a dummy project) but it will give them the right to see each other everywhere else! You could even given them a “comment-only” permission on the project, preventing them from actually leaving the project 
Smart, innit?