Flying coworkers with a commercial pilot's license (USA/FAA)
Since you have a CPL, you can receive compensation for flying.  

Your question and concern is over the means of reimbursement which may have implications beyond that of the FAA. That said, I'm not a lawyer and I would suggest your company consult with one and establish a policy. Most companies I've worked for prohibit personal flying incidental to the business specifically due to liability concerns. 

To avoid confusion, I would stick with option 1) as that makes it clear it's your choice to fly and not that of the business. If the company rents the plane, it may expose the company to additional FAA regulation as well as liability and tax implications.

As a point of explanation; before I retired from the USAF it was not uncommon to use private aircraft to travel on TDY (Temporary Duty - the military version of a business trip). The rules under AF regs and the Joint Travel Regulations (DoD) allowed you to travel using a privately owned aircraft or a rented aircraft. IIRC the AF required a CPL/IR and a total of 200 hrs. The rules also specified what expenses were allowed and how they were to be reimbursed. 

If you used your own aircraft, it was classified as POV (Personally Owned Vehicle) travel. That was no different than driving your own car on the trip. In that case you were reimbursed at the POV mileage rate. That rate when I was in was actually less than the IRS allowed mileage rate and wouldn't come close to covering the cost. It also didn't matter if anyone was travelling with you, be it a coworker or family member. 

If you rented an aircraft, it was treated the same as if the company allowed you to rent a car to make the trip. You were reimbursed your actual rental cost (wet) up to the cost airfare for the trip for all persons travelling together. What that meant was that if you traveled alone, the reimbursement was typically less than the rental cost (say \$400 for airfare vs \$600 rental). If you had two or more travelers, the airfare typically exceeded the rental cost and the entire rental was covered. The limitation was that only coworkers on travel orders counted towards the reimbursement.