welcome to our group Ian,
i am not sure do you want the drop down in the infopath form or on some sharepoint page.
If you want it in infopath form, you can open a data connection to get data on the field (promoted one)
then go to drop down properties, in listbox entries, select look up values from external data source,
select the data source (data connection you just made)
in the entries click the button next to the field, select the group, and use filter option to filter the data.
i guess this should take care of your problem