Hey Patrick,
It did work. Here is the formula we used:
xdMath:Eval(xdMath:Eval(../my:AlsoNotify/pc:Person[pc:DisplayName != ""], 'concat("-1;#", pc:AccountId, ";#")'), "..")
We set this as the default value on a text variable. We also had to put in the filter [pc:DisplayName != ""] because if nothing was entered into the PP field we were getting errors when trying to submit.
Hope this helps others. Thanks