That's a good warm-up. Have all, then a few at a time sing something simple (like Amazing Grace, or Alleluia) in different keys. Listen for the point where their voice cracks, gets softer, strains or goes off-pitch.
The guys who can sing the highest are Tenors, the gals Sopranos - they usually end up doing the melody on Praise team songs. The low-singing men and women are Bass and Altos, respectively, and can sing the melody an octave lower, or be trained to do harmonies (that will probably take some time and work). Some lower-pitched women can also sing Tenor.
Most people are medium range, between Soprano/Alto, or Tenor/Bass, and can find a tune in a congregational range of a little more than an octave (from about Bb/C --->D/Eb)
Any wider than that and you're getting into small group/solo material...