Micheal LeVine, a 66-year-old research physicist from Port Jefferson, N.Y., says that after his nephew invited him to join Geni, he continues to unearth new family connections. "I found relatives I didn't remember I was related to. I went to visit somebody's goat farm when I was 7, and all of a sudden, their name pops up," says LeVine, who finds that he visits the site several times a day to check if his family members have responded to his invitation. During a coming business trip to Chicago, he plans to meet up with a cousin from his mother's side, a woman in her early 90s who is a concentration-camp survivor.

Family tree sites are attracting a young, more global audience than some similar sites: more than half of Geni's traffic comes from outside the United States. Indeed, a problem with the traditional sites is that they worked better for families that had come to America generations ago and less well with more recent immigrants. Meanwhile, the Generations Network also is developing internationally, launching sites in the United Kingdom, Canada, Australia, Germany, China and Italy.

But, some users worry that Geni is too wide-open. "This collaboration thing has some dangers," says LeVine. "I invite people; they can invite people ... and they have the same privileges as I do to erase names or change things. All you need is someone who is careless or malicious, and all your work goes down the drain."