Got this to work. Thanks to Mr. Clayton. Here's what I did.
1.) Created a contact selector called it: RR_Members, created another text control and set it's default to be this xpath expression => xdMath:Eval(xdMath:Eval(../my:RR_Members/my:Person, 'concat(my:AccountId, ";")'), "..")
2.) Finally promote the text control to a Text column in SP. Have the workflow, set the People field to the value of the Text column and it works well.
Assumptions:- Create a group called "Person" and a field called "AccountId" under it, nothing new here.