lots of shade, and stick away from the posed shots. as far as the group shot, put the happy couple right up front and surround them with the people that love them. I by no means am a "professional anniversary photog", but it's all about capturing true, emotional moments. These are pictures that in years will be PRICELESS for the entire family, and will be some of the only visual memories hanging on their walls.
here's a 60th wedding anniversary I shot back in march:
http://jrowephoto.blogspot.com …-wedding-anniversary.html
it's by no means the best pictures out there, but I liked to just sit in the back with my 85mm f/1.8 and snap away while the family interacted with the couple. the more distance, the better. i used my 70-200mm f/2.8L IS USM a few times as well. the more you're away, and out of their minds, the more normal they'll act and you'll get some WONDERFUL shots of them interacting.
good luck to you, can't wait to see how it turns out 