"I walked over to Jessie and gave her a big hug," UN Foundation executive Elizabeth Gore, who also made the hike, tells PEOPLE via satellite phone. "The two of us embraced and we just cried. Every single one of us was in absolute tears."
"The temperature was zero degrees and so many people were struggling," she adds. "People had hurt limbs, people had altitude sickness."
After six days of climbing, the final stretch of the mountain Tuesday proved to be the most arduous. "We had a complete whiteout," says Gore. "There was so much snow, we could only see two or three feet in front of us. So we had to line up like a chain to make it to the final point. I'm so proud that every single one of us 12 climbers made it."
Once the group of climbers – which included Ethiopian-born Kenna and environmentalist Kick Kennedy, granddaughter of Robert F. Kennedy – reached the summit, they quickly had to begin their descent.
The way down is so steep," says Gore. "And we had ice, so you're stepping sideways, trying not to slip. And we hit big boulders, so we had to hop-skip. We had major knee issues."
After eight days of hiking, the group finally reached the town of Arusha, Tanzania at the base of the mountain on late Wednesday. They planned to celebrate with a hot meal at a local restaurant and to toss back a few bottles of the local Tanzanian beer.
On Thursday, Jessica Biel, Kenna and rapper Santigold will join Gore on a trip to a United Nations camp in Ethiopia to see some of the world's most significant water problems. "We are all so tired, dirty and bruised," says Gore, "but we re trying to give a voice to people who don't have any."
For complete coverage of the expedition visit www.summitonthesummit.com