Vocals

Probably the most important aspect of any groups performance.

Everyone wishes for an artist with good mic technique, but read the hints in the mic section so you can 'suggest' techniques to them.

Remember your compression, as most vocalists cannot maintain a constant 'level' of volume. Gates and especially reverb are very important, too.

Engineering

the entire vocal range is about 80Hz to 4kHz, but the voice rarely extends beyond 1kHz, the rest occupied by harmonics and nuances. Be careful to avoid sibilance (excesssssive sssssssing). Avoid EQ'ing at all, if you have to, avoid boosting 1-2kHz, except for FX. 
If they're sounding really 'alone', boost 3-4kHz an ickle bit.

Generally use a Cardioid mic to reject other on-stage noise, although, in a quiet studio, an omni-directional will almost always perform better.

Sibilance

If becoming a huge problem, or in the studio, one method is to get the singer to sing over the mic, rather than into it.

BACKING VOCALISTS

Need to be recessed into the mix, rather than brought forward (as the lead is). Roll off bass and treble a little (opposite of LOUDNESS) to reduce the presence.