Nintex Workflow Cloud - Signature Block

It sounds like you are asking about the signature control in NWC forms.  You could assign a task to an outside party using a form with a signature control.  To do this, in the "Assignee" section, set the "Assignee authentication" to disabled.

While this is fine for capturing simple signatures, if you need to obtain a legally binding signature (like on a contract) you would want to use your AssureSign product.