How to create a picklist from a multiple select lookup?

How to create a picklist from a multiple select lookup?

I am using two forms.  Contact form with the usual fields and an invitation form. I have a two fields in the invitation form.  One field is a multiple select lookup field so I can choose the related contacts for a particular invitation.  The second field I want to pull all the addresses of the respective contacts of the first field and be able to select just one address to send the invitation to. 
 
I tried using the following script but run into an error saying the the left expression is a string and the right expression is a stringlist.  Obviously a mismatch.  Is there another way to do this?
 

If((count(Contact[first_name ==input.first_name])==0

{

Clear Who’s Address;

}

Else

{

Rec = Who’s Address[first_name==input.first_name] sort by first_name;

Who’s Address :ui.add(rec.first_name.getAll());

}