I get it. Here's what worked for me -
concat("Insert /parent=/my:myFields/my:Rooming/my:HotelGroup/my:HotelRepeatingSection[", count(preceding-sibling::HotelRepeatingSection) + 1, "]/my:RoomingGroup /xpathsrc=/my:myFields/my:Rooming/my:HotelGroup/my:HotelRepeatingSection/my:RoomingGroup/my:RoomingTable")
Now (and I'm trying to be self-sufficient here, but . . . ) how do I accomplish the same with a remove button that resides within the repeating table (Rooming Table)? Here' what I have in there now -
concat("Delete /xpath=my:myFields/my:Rooming/my:HotelGroup/my:HotelRepeatingSection/my:RoomingGroup/my:RoomingTable[", count(preceding-sibling::RoomingTable) + 1, "]")
It removes the last row of every instance of the repeating table (within every instance of the HotelRepeatingSection)