If he has the individual know how he could modify human sized radios to do the broadcasts he needs.
If he knows the general area his people are supposed to be in he has more options. Especially if the area is an acre or less.
He could visit every ant hive within the acre and paint a quick message on each ant using a stencil as they go out.
Does he know Bill prime is studying a certain item? Perhaps Bill Alt can focus his communication vector on that. Bill Prime studying flowers? Then taking Bees might be more effective.
Does Bill Alt have to cover a larger area? If he can capture and deploy a remote drone, he could have it carry around an IPOD reprogrammed to blast out some of the more popular hits from back home on planet Bill.
Is Bill Prime studying an aspect of human culture? He could insert subtle references to the culture of planet Bill into the media.
Many more options are available depending upon his personal knowledge and skills and what the other Bill's are studying.
Technique for larger areas
If Bill Alt needs to track down his people from a larger area and his people are not prone to looking up from their studies then audio is probably the best method to broadcast over a large area.
Accessing Emergency broadcast systems for cites and counties and have them broadcast a message in his native tongue is probably the most efficient way here in the US. These systems are not universally set up though and would require different skills. Some are simple analog alarms, and would need a physical modification of the alarm sirens. Others are digital, and could be as simple as overwriting a file in a computer system (if he can access it).