Would it really give positive assist for a person that was off while the rest were carrying key/pitch?
No. That would merely amplify their own voice at the expense of hearing the other singers who were singing in tune. This is not a simple solution question. My strong belief is that the person/s in question would benefit from working with a smaller group, say three or four singers, who then, as a small group, could explore and practice the challenge of singing together and hopefully would gain a better understanding of the dynamics involved.