I have been to Bandon Dunes seven times since 2005. There are no right or wrong answers, but my opinion is as follows:
1) The best chance to avoid rain is to go in June, July, August, or September. It does not rain much in the summer months, but the wind blows harder in my opinion. The longer days in June and early July allow for more rest between rounds on days that you play more than 18 holes.
2) On your first trip to Bandon Dunes play every course on property. There is not a single best course, but rather four great courses. In your group of eight I would wager that every course offered will be someone's favorite.
3) The food is pretty good all over the property. Trails End is excellent for lunch before playing Bandon Trails. The Pacific Grill is excellent for dinner and serves an excellent breakfast wrap. Pacific Grill is also a great place for cocktails and a match at the Punch Bowl. McKee's Pub and The Lodge are also fun for lunch and dinner. With a group of eight you will want to make restaurant reservations for lunch and dinner as the venues are not very large and can become very crowded.
4) Take a caddie! You are on vacation and there are some very good loopers at Bandon Dunes.
5) Do not miss playing Bandon Preserve as it is a really fun Par 3 course.
6) Personally, I like playing Bandon Trails in the afternoon to get out of the wind as much as possible in the trees. I like playing Old Mac in the early morning before the wind picks up as it is the most exposed in my opinion.
You and your group will have a great time!