Westech has it correct! Although you may think it's o.k. to just "join in". The convoy commander has a specific number and types of vehicles alloted for the convoy based on details submitted to his/her Department of Public Safety. They all MUST carry a convoy number, and they must adhere to a HOST of other rules to be able to move the vehicles.
The rule of thumb for us while I was in, was simple. If a civilian vehicle "accidently merged" within the convoy, the lead vehicle would slow down to 35 - 40 mph.... coordinated through the radio so we didn't cause a pile up. The civilian then finding out it wasn't "that cool" would unmerge themselves and go about their merry way. The obvious other issue is the fact that the BIG trucks don't stop all that well and the thought of a camry, lexus, corvette or other little vehicle getting sandwiched between two 5 tons was never appealing.
So, if you ever want to join a "REAL" convoy..... SIGN UP !!! Uncle Sam stills wants YOU.... he had me for 21 plus years.....