To coordinate multiple people, a great strategy is to pick one patterned item of clothing, then choose other outfits around that.
Try to avoid having everyone in matching outfits (i.e. everyone in white polos and jeans)... that trend was popular in the 80s, but doesn't help celebrate that uniqueness that you love about each member of your group.
If you have a particular family member who you want to emphasize, put them in something that CONTRASTS with everyone else. For example:
Saturated, rich colors look gorgeous in outdoor settings.
Consider what colors predominate in your setting, and choose YOUR colors from the OPPOSITE side of the color wheel.
(Ex: rich blues and purples look amazing against yellow fall leaves and dry desert grass; reds, oranges and pinks are great choices for green settings; and any vibrant, intense colors look great with sagey or snowy backgrounds.)
Do a little backwards planning and consider choosing outfits that complement your decor!