Hi @Lachlan and welcome to the Forum!
Although it is possible to designate a default assignee for each form submission, as it stands, it is not possible to automatically add a user as a collaborator of the task created from a Form.
There is an existing thread regarding this topic in the Forum: Automatically Adding Task Collaborators via Asana Forms. I would suggest you to upvote it!
@Phil_Seeman: is it possible to achieve this with Flowsana?
Hope this helps @Lachlan!