You may not know it, but your value to a band will increase dramatically if you can help with the vocals. There are guitar players on every street, but only a fraction can sing and being able to support the band with harmonies makes you that much more desirable. If you listen, really listen, to a lot of your favorite songs, you'll realize there are a lot of harmonies going on, especially in the chorus. I believe drummers get out of the task easier, so it's mainly guitar, bass, and keyboard players helping out. The good news is that you don't have to have a great voice to be able to sing harmonies, but having a good ear for music helps.
When beginners get together to form a band, they often try to boost their vocals by having multiple people singing, but instead of singing harmony parts, they sing the same notes. Since they rarely are singing the *exact* same pitch at the same time, instead of a pleasant doubling effect, it just sounds awful. You might not realize it while singing, but ask someone to listen or better yet record yourselves. There's a reason that it doesn't sound like normal songs and that's because you need to learn how to sing harmony parts.
First let me outline a couple of rules for you, the backup singer (which you can break later):
1) don't ever sing the same note as the lead singer
2) don't sing through the entire song (ie, only sing in chorus or emphasis parts)
Finding the Harmony NotesNow, let's figure out how to find the harmony notes to sing. As a guitar player, you're already familiar with chords and intervals. Since you're not singing the main melody note, you have to find another note (rule #1). This will usually be either a 3rd or 5th note above the lead note. Sometimes, you'll need to sing lower than the main note (perhaps because you can't sing the higher note). In this case, you may see the note an octave below the 3rd labeled as a "6th" because that's the interval lower from the root. But it still has the feel of a 3rd in a chord. You'll want to practicing singing both 3rd's and 5th's so you can hear the different feel that each one creates. For the rest of this lesson, I'm going to focus on 3rd's, but everything applies equally to 5ths.
For review, here is guitar TAB for the notes of the key of C with the corresponding 3rd notes above each note. This is taking each note and finding the third note above it (either major or minor third, depending on which note stays in the key of C).
To get your self started on a song, try playing the vocal melody notes on your guitar first. When you have the melody figured out, try playing each melody note along with the 3rd above it (you can use the example fingering from the TAB above). Then play only the notes that are a 3rd above. This is what you want to sing.
Note that the interval is changing because sometimes you are a minor third and sometimes a major third above. If you use a smart harmonizer (either as a guitar effect or vocal effect), it will automatically pick the correct 3rd based on the current musical key. (as an aside - this is how you would create harmonized guitar solos!)
Now, here is the tricky part. You probably don't want to figure out your entire harmony part on the guitar. Just figure out enough notes to get yourself going. Think of it as learning to ride a bike without training wheels. Someone gives you a push and then you just keep going. That's what you want to do here. Once you get the feel to where you need to sing, just keep going - you can get the feel of the interval and just keep singing your part. You'll automatically pick the right 3rd when you are singing the one that "sounds right."
PracticeOnce you've decided you want to sing harmony backups, do it all the time. Turn on some music and harmonize with the whole song (ignore rule #2 above when your not with your band!) When the radio is turned on, try singing harmonies along with it. Even if you don't know the lyrics, try to harmonize along (if you ever saw the show Who's Line Is It? you'll see there are people that can just harmonize on-the-spot as someone makes up lyrics!) If you're girlfriend/boyfriend is talking, harmonize along. (OK, don't really do that!)
While your singing harmonies, be aware of your own singing range. I have a pretty limited range and can sing safely below middle-C (B-string, first fret). After some warmup and inspiration, I can hit the E above that. Generally, singing falsetto does not sound good with the main vocals, but you may be able to use your falsetto voice to add harmonies, or you can sing the note that is an octave below the 3rd note above the main vocals. Just don't plan on jumping back and forth between natural and falsetto voice to cover your limited range.
You might think of singing a full octave apart as a substitute for real harmonies. I'd recommend against it. Singing an octave apart suffers from the same issue as doubling - unless the notes are exactly in pitch, listeners will hear the dissonance. In addition, octaves don't provide the "color" that 3rds and 5ths create.
After mastering 3rds, you'll want to learn 5ths and also be able to add other intervals (6th and 7th's can sound jazzy or bluesy).
Live PlayingThere are two big problems when you go to sing harmonies with your band. Either you'll drift back to the main notes, or the lead singer will drift over to your notes. The objective is to recognize when this happens, then stop and sort things out. After some practice, you will both be able to keep your parts separate. Try having the lead singer go back and sing just his part, then you sing just your part, then try putting them together again.
Make sure that you can adequately hear each other (especially you being able to hear the lead vocals). You may need to ask for some more lead vocals in your monitor mix. You really need to hear both your voice and the lead voice mixing in your head to keep the proper interval.
Using a HarmonizerI also use an electronic harmonizer. I have a Digitech Vocalist Live 4, but there are several other types available. Unlike the older style that required some type of MIDI programming to figure out the key, these new boxes route your rhythm guitar through them so that they can analyze the chords as you play them. Whenever your hold down the pedal, the box adds harmony parts to your own voice and makes sure they fit the current chord (no programming required!). These won't sound as good as good real backup singers - they have a slight electronic sound to them, but if you keep the harmony levels low, they'll add the missing color that your lead vocalist needs.
You can also use the harmonizer to add harmony parts to your backup vocals. That is, if you have a song with a lot of oohs and ahhs, you can have the harmonizer add thick 3rds and 5ths (until you can get the bass player and drummer to help *you* out!).