why is it that cell companies(verizon, tmobile, sprint, etc) share towers, but for example i have tmobile with edge and my friend next to me with verizon has full 3G bars?

 

No idea...

 

If a cell tower is owned by one company, they don't always enable all features for all other companies.

Similar to your calls get dropped more often on competitor's towers than on the towers owned by your company.

 

It could also relate to the devices you use. Some device has a better antenna than others.
...Or maybe there is something the cell cell companies don't want us to know...