Lost and Found was a nine member adventuring party that formed after the group escaped the captivity of the Agents of the Overseer. Some were longtime residents of Waterdeep, others were visitors from far off lands, yet they were all kidnapped or captured and destined to be sold into slavery or worse. They went on to work together on an initial mission helping to rescue Floon Blaagmar who had been kidnapped. They were dubbed "Lost and Found" by Volo after those events.
Volo didn't tell them this when he suggested the name, but it was an acronym for:
League of Slaves Together Adventuring Now Despite Few Other Unifying Narrative Details