For adding users to the hub, the "default_role" parameter defines the user's default role in the account.
You can specify it by adding the "default_role" field when calling POST users/import endpoint.
You should specify in this field a valid name of an industry role available.
One example of a valid call to add a user with default role would contain the body below:
[{
"email": "user email",
"default_role":"some valid role name",
"default_role_id": "some valid role id"
}]
From there, when you add the user to a project from UI, it'll come with this role defined.
When you add to a project from API, the "industry_roles" parameter will define their roles (reference here).