We have a really good lead vocalist/frontman (who can be a bit of a diva, but what's new?). From the rest of the band, one of the guitarists can, sort of, sing but can be a bit shouty and intonation can be suspect - he only joins in on backing vox when we cover Thin Lizzy's 'The Boys are Back In Town" at the obvious place. Our drummer says he can sing, but I've never heard him because he's new into the band, has only done five gigs with us, and says he wants to be on autopilot drum-wise before he joins in with vox, which is fair enough. That leaves me. I can sing (in tune when the monitor mix is as it should be) and good enough to have sung lead vocals in previous bands, although I would be the first to admit that I am a second rate lead vocalist; my range is OK but not stellar and the tone of my voice is quite ordinary. Nonetheless, to date, I am the bands only backing vocalist (apart from the 'boys are back' shouty response).
What I find really helps is that I have a TC Helicon Voicelive Play that I have programmed with appropriate vocal doubling and harmonies for each song. If you're a guitarist it would be great because it works out the song's key and harmonies from the chords you play, as a bassist it's useless unless you double stop the third or minor third because that's how it calculates the harmony to apply, so you have to pre-program the key for each song, then apply the harmony voice or doubling options you want to use....it's got lots of other effects, like loads of reverb options, but I don't use those because our lead vocals use the reverb FX on the mixer (that I also run), so we both use the same reverb send and return so that it doesn't sound like he's in a cathedral and I'm in a small room. Needless to say, like all effects, you have to be very judicious in the harmonies and doubling to use, but once you've nailed it the backing vox can sound very effective...subtlety wins every time.
A guitarist from another band came to see us and looked down his nose at this little pedal, then told me I was cheating and should be able to sing without artificial aids (which I can)...I told him that was an ironic comment coming from a guy who has nearly as many FX going on as The Edge and that he should be able to do it with his guitar plugged straight into an amp...I think he got the point.