Travel + Leisure Magazine: http://www.travelandleisure.com/articles/all-together-now
The American Society of Travel Agents: www.travelsense.org
Travel + Leisure also has al list of travel agents that specialize in family travel and, specifically, multi-generational travel: http://www.travelandleisure.com/alist/2006/specialties.cfm?area=12&by=type
The Fairmont Banff Springs Hotel has an on-staff sales manager who can help coordinate family reunions: http://www.fairmontmeetings.com/banffsprings/property_01.html
Hilton has a special e-events web site to help book group events at their properties: http://www.hiltonfamily.com/en/ww/promotions/learneevents/?WT.srch=1.
Some trip planning sites do a lot of the work for you and make it easy to set up a page --: http://www.triphub.com/usered/tour.aspx