This is a 6 transistor full adder. So far I can't get the count any lower than this but for the sake of pointless curiosity I will try.
The XOR functions are fulfilled using two diodes for an OR and then using two transistors to turn the output off for the AND.
This is done twice for A, B and Carry leaving by consequence A & B
And additionally
(A XOR B) & Carry
Which are subsequently OR-ed using two extra diodes and then buffered with two transistors.
