It all depends on:
1. The ticket counter people. They need to let you get a pass through security.
2. The gate agent.
3. The captain.
If you can make it through the first two hurdles, you will probably make it past the captain. You'll be sitting in the back anyway.
You need to have a company ID, pilot certificate, medical.
It helps if you are a pilot for a larger, well known 135 operation, such as Airnet.
And yes, you can ride on 135. Back in my 135 freight dog days I actually took some pilots to work who could not get on the 121 flights. Kind of sucked sitting in a Sennaca with a bunch of boxes, but I guess it was better than calling in sick. It helped that we had scheduled runs and the local pilots knew about us.
Also, as some have pointed out, do NOT use it for company business.