Op amp Adder circuit and positive voltages
This circuit will not work with the negative supply pin connected to ground.  It is an inverting circuit, which causes the input voltage to be "mirrored" around the + input, in this case 0V.

But the opamp's output cannot go beyond the supply rails, which are at 0V and +9V.

To fix this you can connect the + input to 4.5V instead of 0V. (This assumes Rf is equal to R0..Rn)  One way to accomplish this is a voltage divider, equal resistors to +9V and ground.