Hi @Ashani_Gottlieb & welcome to the forum
I’m afraid there is no way to show only the individuals belonging to a project in the assignee field because, in Asana, anyone can be assigned to a task regardless of their permissions in the project itself.
I could see how this could be helpful and something nice to be added to the project settings. I would upvote this feature here: Assignee field should only show members of a project or team and leave feedback on this thread for your specific use case.
Best of luck with your project!