Out the box, subpanels are build as relationships to other modules.
To create a subpanel you first need to create a relationship you can do this in Admin > Studio > Accounts > Relationships
Then add the relationship with the desired type. You may have to build a custom module however depending on exactly what you want. (Accounts has an ‘assign to’ field already)
A basic description of the types is:
One to one = Relate field on both modules
One to Many = Relate field to Subpanel
Many to Many = Subpanel to Subpanel