Many CTAF frequencies are also UNICOM frequencies on uncontrolled airports. This essentially means the FBO or ground operation there will likely be monitoring, and is authorized to talk back, on such common advisory frequencies. There is no license requirement forbidding someone without an FAA license from using a UNICOM freq to talk to air traffic in their local area, however the ground station will have to have an FCC license to do so.
As far as a pilot giving traffic advisories on the ground, I've heard a multitude of different things on CTAF freq's over the years. Since CTAF's aren't typically monitored by ATC, and not many rules governing them, you can hear things like casual conversations between pilots, traffic advisories, position reports, even pilots on the ground telling incoming aircraft about potential hazards that might exist in the area (traffic, fires, obstacles, etc.) However, there isn't a FAR that specifically dictates what communications are forbid or allowed on a CTAF. The AIM (Airman's Information Manual) suggests procedures when using a CTAF, but the gist is whatever is necessary to conduct a safe operations in the vicinity of an uncontrolled airport. The other however is there is actually no reporting requirements for operating at an uncontrolled airport in class E or G airspace - in other words, it's completely up to the pilot to do so. It would seem like a no-brainer to announce position reports, but you'd be surprised at the things you encounter at uncontrolled airports...