Insurance companies conduct a household report as part of their underwriting process, just like they run your Motor Vehicle Report and CLUE (Comprehensive Loss Underwriting Exchange) to determine "potential" exposure. In your case, it sounds like they found some people that were not listed on your application. Sometimes, these are people that lived in the house a long time ago!
Normally we will send out a letter letting you know about this and ask you to come in and sign some papers to add or exclude them from your policy. If you don't reply, we can't reply to the insurance company and they will automatically add them to your policy and make your rates go up - especially if there is a youthful driver in the house. I have seen companies that want to exclude anyone 14 and over!
In some States, the definition of insured are 1) the named insured 2) spouse and 3) residents of the household. So, if all of these folks are insured by definition, the insurance company either wants to list them on your policy (and charge an approriate premium for the risk) or exclude them.
Just tell your agent that you want to exclude these other drivers. If they don't want to do it or can't, look around in your local area for an independent insurance agent and call them. Just look in your phonebook for the PIA or Big I (Trusted CHoice) logos and you will find a professional agent there that can help you. An independent will normally have a dozen or more different companies to choose from to help solve your insurance needs.
Good luck, drive RESPONSIBLY and I hope this helps!