Turn off all the music. Play only the lead vocal. Do not pan this vocal to the right or left in the mix, put it right in the center. Get all the tones you want in the vocal, and add the reverb that makes the singer sound like you want him to sound. Do this with no instruments or background singers playing.
Boost the volume slowly on the first background singer, while listening to the lead singer. Remember that you will be adding other background singers, so don't make the first one too loud. Bring up the background singer to a moderate volume level. Pan that singer to the right, at about 2 o'clock if there were a clock in front of you, and the lead singer was at 12 o'clock.
Raise the volume on the second background singer. Do this while listening to the lead singer and the first background singers. After reaching a moderate volume that doesn't overpower the lead singer, pan the second background singer to the left, at about the 10 o'clock position.
Repeat the process for each additional background singer. Pan each subsequent singer to the right or left until you have a balanced "fan" spread out behind the lead singer. Example: Lead singer at 12 o'clock. then background singers at 9, 10, 2 and 4 o'clock.
Step back from the speakers and listen to the relative volume of the background singers compared to the lead singer. It is very common to become focused on the background singers too much and make them too loud. Make sure the lead singer "cuts through" the wall of sound the background singers put out.
Bring up the instruments. The vocals should be the main focus of the recording, so get rid of any instruments that make the vocals sound muddy or have conflicting notes. Bring in the instruments in such a way that they support what the lead singer and background singers are doing.