Arrive early at the venue and see what type of acoustics the room has. Bare walls and floors will usually create a lot of reverb, while a well-carpeted one may not.
Set up the PA with the effects that you will be using by patching them into the "effects" sends on your mixing board, unless your board already has built-in effects.
Do a sound-check with the performers to see what it sounds like in the room. Remember that people's bodies will also tend to absorb sound.
Try putting carpet or a blanket up on the back wall behind the sound board to absorb some of the sound if there seems like there is too much echo.
Find out what type of sound the performers are looking for on each song and take notes. If the room is dead sounding, then a medium or large reverb may help brighten up the vocals or the acoustic guitars.
Use a chorus effect, or "choralizer," to create the illusion of many voices singing, or flange for a phasing or "otherworldly" effect. The speed and depth of the chorus or flange can be controlled to change the sound.
Give the vocals a more "in your face" feel by using a compressor to "squeeze" the sound. A noise gate can also help in live situations by filtering out unwanted background noise such as amplifiers from coming through the vocal mic.
Have each vocalist sing and adjust the effects as desired. Try out different effects for different songs, adjusting the speed and depth on each one until the desired sound is achieved.