Steven Greer was most recently known for his documentary film Unacknowledged, which covers the entire history of UFOs and military cover ups since WW2.  The documentary has a number of credible witnesses that are retired military etc.  He also runs CSETI and does CE5 Tours (Close Encounters of the 5th kind) where they proactively make contact using the CE5 protocols, and UFO sightings follow in response.

Steven Macon Greer (June 28, 1955) is an American ufologist and retired traumatologist who founded the Center for the Study of Extraterrestrial Intelligence (CSETI) and the Disclosure Project, which seeks the disclosure of allegedly classified secret UFO information.