Group Portraits are always fun and interesting to do!
As far was posing, the photographer may position you all to convey a certain message. Or he/she could have all of you in several different positions as a means of identifying his/her state of mind an character.
If a photographer places you or a family member in a certain position that you question whether it will look good or not, if you have a hired a a photographer with much experience of portrait taking, then i wouldn't worry about it too much. They're probably trying to adjust them due to their (as i like to call them) People Problems.
For example, People with round or flat faces look best when photographed in short lighting or posed 3/4 to the camera. Others with wrinkles or deep lines look better when photographer in a softer light. Other with larger noses should be positioned with their chin raised slightly, or posed with their nose straight into the lens. People with different sized eyes are interesting to photograph; Largest eye close to camera, or largest eye away to create an evening effect.
Some basic positioning tips are:
*Body and head facing different directions
*Men, for a more masculine look, tilt top of head toward far shoulder
*Women, for a more feminine look, tilt top of the head toward near shoulder
*Don't show flat surfaces
*Fingers SHOULD NOT be facing into lens
*Male hands, more closed
*Female hands, more open
*Not intertwined or in a fist
If your family members such as, a married couple, would like to photographed close together in the group portrait, either have them facing front to front, or fronts facing forward, and avoid pace between heads.
And some stuff to have on handy are combs, mirrors, hair spray/gel, hair clips. etc.
And last thing, if you are photographing in a studio, be careful about lighting. You don't want to wash away details on your subject, or their face! If you are photographing outside, have the flash on, for fill lighting. Yes i know, there is the sun, but people do not look good at all under direct light, it creates weird shadows on their face. If you photograph outside, have your subjects in some type of shade, like under a tree, and use the flash for Fill Lighting, for a nice glow on your subject's face.
Hope this helps you! :)
In Digital Photography Class
Mother Professional Photographer
Much experiences with this on my own :)