Before employing some electronic means of pitch correction, you should ensure that the off-key vocalists are able to hear a clear reference of the note they are expected to sing.
For example, some people have a difficult time picking a single note out of a chord. Usually it's fairly easy to hear the melody (soprano part), but the alto, tenor, and bass parts are more difficult to discern. If you can hear the note you are trying to sing (e.g. all the tenors can hear the other tenors) then you'll have a much better chance of singing on-key.
If you can provide your vocalists the means to sing correctly, it will sound a thousand times better than anything you could hope to achieve with pitch correction.
I have no experience implementing pitch correction, but I have heard a lot of music where it's obvious that pitch correction has been employed and it's very annoying.