Dears , 

I have seen couple posts asking about having an activity with dynamic destination user defined by the previous activity user , ("To have a certain activity with Datafield that holds username defind by the previous form , from a picker control , to be set into its "Destination")

the scenario is , if you have a smartform that wants to initiate a workflow process , and you want the user to be defind dynamically for the first activity , either from sharepoint groups or AD users.

i fill my form , choose who is the username i want to associate the activity to , on my submit action.

you could follow the document hoping to help u find the right approach .

Regards.