What is the best time of day?
My favorite time of day to take pictures is usually an hour before sunset, the light is just right then and the photos get a nice warm look to them. However during the day is ok, but the best pictures are in the shade then so that the shadows are not to harsh.
What is the best location?
I am personally pretty indifferent actually for location. I usually gravitate towards being in a forest, field or park because of the open space and greenery. Any where a group can fit is ok with me though.
What to wear?
Generally speaking, plain dull colors and layers are best. In a group picture patterns and bright colors can be a bit distracting, so I recommend trying to match with a general color palette, such as red, blue, brown and white or something similar, so as a group everyone looks nice together and no one really seems out of place.