Numbers Sum of all values in a column where another cell within the row matches text
I've found it! 

I opened the function browser and read the description of every function down the list until I finally found the SUMIF function!
SUMIF(table1::ID, $A2, table1::numbers)

The first parameter creates a range across the whole column with header ID in table1, the second is a strict column reference with a relative row reference, so I could copy / paste down the whole column, and the third parameter is the entire column with header numbers in table1.