Hi,
According to your post, my understanding is that you wanted to store a User on InfoPath Form and utilise this Variable in a Workflow.
If you create workflow associated to a list, when you customize InfoPath form, once you add the user filed to the form, the user filed will be avaliable to the SharePoint Site.
If you create workflow associated to a library, when you publish InfoPath form, you need to make the user filed avaliable to the SharePoint Site.
Then in the workflow you can get the InfoPath filed value as below:
Thank you for your understanding.
Best Regards,
Linda Li