Hi!
I'd like to grant JUST some users, set as "collaborator" in my project, to create sub-task (so not other issues).
Reading your form, I tried the option to create a specific workflow JUST for my sub-task (so the other issues have the standard workflow that allows just people with the create permission to create the issues), and I set the create validator to those people that have "Assignable User" permission in my project.
Unfortunately, it doesn't work.
Can you please confirm me that is NOT possible in this way?
I have Cloud Atlassian 
Thanks a lot for your help!
Fiorella