Another requirement for my teleconference reservation form was to only allow one unique user to be assigned to a number.
I had to figure a way to filter or exclude previously selected users from other drop-down box selections.
My user drop-down is populated via the Attendees people picker field from the Main datasource.
I then created a filter on the Person group datafields to limit the drop-down options displayed.
The actual expression is:
not(pc:DisplayName = xdXDocument:getDom("ConferenceMaster")/dfs:myFields/dfs:dataFields/d:SharePointListItem_RW/d:User)
The resulting form looks like this; Only when Assign is clicked do the following drop-down boxes filter.
For reference, my Assign button rules are depicted here: