May I know why you don't want to make them followers of that project, is it because of they can see all the project details and other followers tasks etc? or anything else?

Yes that is basically it. We have many project participants and I don't want them to be able to 'see' the whole project and all the steps.

I found a different approach in a stackoverflow post: "Tasks should show up only if the user has been assigned it" using record rules. Still tinkering with it to make it work.

object/model: project.task

name: See own tasks only

domain: ['|',('user_id','=',False),('user_id','=',user.id)]

(means: your own tasks and unassigned ones)

apply for read: [x]

apply for write: [x]

apply for create: [x]

apply for delete: [x]

groups: Project / User

Still trying to make it work, though.

Thank you to the original poster.