I normally create a concat names field that sets the People Picker values to a regular text field. If you are using claims based authentication, the email addresses are not formatted in a simple [email protected]. The email addresses are formatted liked " i:0#.f|membership|[email protected].
Create a text field that will be used to concatentate the names that are entered into the People Picker. Have a rule that sets the fields value each time the people picker is changed. You will need to strip off the beginning part of the email address and add a semi-colon in between each name. The code would look something like this:
xdMath:Eval(xdMath:Eval(../my:group/pc:Person,
'concat( substring-after( *[local-name() = "AccountId"],
" i:0#.f|membership|"), ";")'), "..")
You can promote this field and use it for sending purposes.
BTW, you cannot enter groups in the People Picker field using this method.