The more people that get on WiFi in my neighborhood the less strength I have... Comcast says it's my router.. I have an Extreme Airport router. What is going on ?

Well - Comcast is taking the easy way out by pointing to your router. The Internet services that Comcast provides is a shared media design - the more people on your segemnt the fewer resources are available. Tryrunning a speed test to see what Comcast is providing. You can even use Comcast's own speedtest site http://speedtest.comcast.net. Once you determine what speed or bandwidth that you are provided then you can determine if changes are required in your Airport extreme such as location of internal option setting.

It appears that Comcast is providing (at the time of the speedtest) excelent service. Can you quantify your refernce of "strength". What are you using to determine strength?