First you can do the pitch and pan change as suggested by everyone else. Pan a copy of the master take left, pitch shift by 40 cents and delay by 15ms. Pan a second copy right, pitch shift by 30 cents and delay by 25 ms. Now you've got your main stereo field which you only need to make more dense and add depth.
Make more copies and apply pitch and formant shifts (I love Logic's vocal transformer for this) and pan them around and probably drop them in level too. Some EQ on each of these tracks might help clean things a bit.
Send all copies and the main take to a group channel (or Aux in Logic/PT). Drop a reverb plugin (a good IR would be nice) that can mimic the space well. Once you get this sound 'almost there' use a send on this group channel to send this signal to an FX Channel (Aux in Logic/PT). Drop a delay plugin on this FXchannel with around 10-15% feedback and delay time of 25-40ms (varying amounts on left and right). In Nuendo change the panner to the "stereo combined panner" and swap left and right channels. EQ and level this channel to 'taste'.
You can control the amount of density. This worked for me well just last week when we had to re create a crowd singing.