One issue not mentioned yet--make sure the area you're going to (with which I'm not familiar) doesn't have restrictions on party size. Most of the places I backpack (in the mountain west) have party size limits of anywhere from 12 down to 8. IMHO, 20 is an extremely large group. You'll have to work extra diligently on "Leave No Trace" practices just to keep the place you're visiting from looking trashed.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey