First thing I would do is solo each vocal track and EQ out all frequencies that don't affect the character of the voice, in other words get rid of all unnecessary content. It's additive when you mix them all together.
Maybe everything below 250Hz on females and 100Hz on males (unless it is a deep male voice).
It really is something you have to tinker with. I had a female vocalist that was not happy with the sound of her voice on her recordings, and by boosting 330Hz a little she suddenly liked it.. she said it put back the sound of her voice that the recording had lost. With Barry (a vocalist I work with often) I cut 330 quite a bit when I need his voice to come through better.
Working with multiple voices I would treat each one individually so their character still has the punch to get through at lower volume, without all the frequencies that are adding the mess.

The old 'trust your ears' approach.

I do not work here, but the benefits are still awesome
Make your sound your own!