HOW TO SING BACK UP IN A BAND
I’m not a singer, but I have played in bands and have tried to sing. Most of the time, it sounded bad. It wasn’t that I am a bad singer, although that probably contributed to the situation. It’s because singing at a club in front of loud instruments calls for some adjustment of technique. For the first year of playing out at bars, I would sing backup, but it wasn’t really being heard in the room. Then there were the times, that I felt I was on key and in tune, and yet, stuck out too much. I think that at the core of it, the technique of singing into a microphone in a band situation is different from say singing in a choir, or in the shower. Microphones accentuate certain aspects of the sound, and reward for crispness of the sound coming out of your mouth, rather than the tone coming out. Like I said, I’m not a singer, but I have noticed some things here and there about singing backup that might be good to keep in mind….
SING LIKE YOU MEAN IT
So, my problem when I started was that I was focused on “blending” which is a term used for choir singers to match the tonal quality of the person next to you. Thereby giving it like a more “rounded” or smooth sound. When you add 50 people to it, it sounds large and reverby.
However, when you use that technique for rock singing into an SM57 or some other microphone, all that comes out is this wash of a sound. I think that onstage microphones used for rock, or hip hop, reward staccato ryhthms and “sharp” sounds. Sounds that are able to cut through the loudness of the stage itself, and the din of live music. If you have a band, and the bass is taking care of the lows, and the keys, and guitars are in the mids, and cymbals are in the highs… it really doesn’t leave very much frequency range for the human voice (which reside somewhere in the mids). So, “blending” to get a round sound doesn’t do very much except add to the wash of sound.
I believe that the correct path to take is to sing as if you were the lead singer with your parts. With conviction. And try to produce a tone that is clear and bright. The blending should be taken care of by the reverb and other effects of the PA system. One important note about that is that it’s important to be sure that the rythms of the melody are in the right place. That way, things will sound locked in.
PUT YOUR MIC WHERE YOUR MOUTH IS
Another common mistake that I see people do is to sing “across” the mic, rather than into it. In the rush to setup for a gig, in New york there is like five minutes of time from when the other band leaves to when you have to go on, I’ve seen some people point the mic to set it up and then leave it for the rest of the show as they play their instrument and sing “under” the mic. Maybe pointing the mic at their eyebrows or something.
Every mic has it’s sweet spots for where it picks up sound the best, but to make it simple, in most live situations, you’ll find a mic not unlike the Shure SM58 or something like that. 
So, Just keep in mind that you want to point your mouth directly into the mic. Pretend you are singing into the bottom of the mic. As if you were singing into a paper towel tube. If that means adjusting the mic on the stand and at a lower level than you may be accustomed, then so be it.
I think you’ll find the sound more to your liking. Good luck.

