first of all you need at least 3-4 people.
1. put them in front of a (good) mic and make them sing the chorus part.
2. then, make them sing it again, in perfect sync with the first time.
3. pan these two parts hard left and hard right.
4. add some hall reverb to taste and listen.
if you wish to have more "crowd" effect, repeat step 1 and 2 changing their disposition in front of the mic, but this time don't pan hard left/right. try with 70% left and 70% right, so that you can have 4 singing parts that fit in the whole stereo field and to avoid phase problems.
the key is to get every time a slightly different singing line, but always in perfect sync.