Demodulation of OFDM symbol in LTE for multi-user case
The zero padding (or the cyclic prefix) takes care of inter-symbol interference that occurs due to delay spread. It is hence something you append to your signal in time domain. Hence the order is this: 


Assemble all the data symbols along the subcarriers (can contain data for multiple users, as you said).
Compute DFT (via FFT, typically)
Add cyclic prefix (or zero padding)


Then transmit.