Is there anything in the FARs that would prohibit a commercial pilot from doing multiple trips to ferry friends to an event in his/her personal aircraft assuming that the friends did not provide any compensation?
If there is no compensation, even a private pilot can do this as much as they want. If the PIC is paying all expenses, they are not receiving anything of value from anyone except themselves.
"Logging hours is compensation" is true in the sense that it can be considered to be a form of compensation when the pilot is not paying all operating costs.
Note that compensation is not necessarily cash. The FAA interprets compensation very broadly. For instance, you may be in violation if you give flights to business partners with the reasonable expectation that you will receive more business from them in the future. This isn't a problem if you're flying friends, though.
According to this interpretation that seems to match your scenario very closely, as long as you're also attending the event then a single trip is fine but multiple trips are not. And you would be acting as a private pilot, not a commercial pilot.
The interpretation was about an ATP who ferried fellow club members to an event (in nine flights!) and did receive money to cover costs, but the main points would apply to a CPL in your scenario too. In summary:
- Flight time is compensation, therefore you're being compensated for the flights even if no money changes hands (the ATP argued that flight time in a C172 was worthless to him, but the FAA said they don't analyze individual cases)
- Acting as a private pilot you can receive compensation only if a "bona fide common purpose" exists
- If you aren't attending the event yourself, there's no common purpose
- If you are attending the event, only the first flights there and back have a common purpose; subsequent flights to bring more people to the event do not
- Acting as a commercial pilot you can receive compensation for transporting people only if you have an air carrier certificate (per 14 CFR 119.1)
You didn't give many details about your scenario, but if you're attending the event yourself then you should be fine for one flight each way under your private privileges. Any more than that and there's no common purpose.
Of course, the caveat here is that every situation is different and perhaps there's something about your case that would result in a different FAA interpretation. However, the basic points about compensation and common purpose are very consistent across interpretations.
(Apologies for not quoting the interpretation directly, but the FAA seems determined to make it as difficult as possible to copy and paste from their documents.)
Citing this similar case, if the pilot shares a common purpose with the friends, there seems to be no issue if it is one flight.
Assuming, however, that the pilot planned to attend the race and conducted the flights with this purpose, his bona fide common purpose with his passengers for making the trip would only extend to the first flight to the race site.
If in doubt, as such matters confuse many commercial and airline transport pilots, contact the local FSDO for their interpretation of the intended operation.
Plane type, weight, and passenger capacity also play a role in the legality of the matter. Even if no money changes hands, certain scenarios require an air carrier / commercial operator license.
Is logging hours a compensation? Yes.
The FAA has consistently construed compensation broadly. Compensation "does not require a profit, a profit motive, or the actual payment of funds." Legal Interpretation to Joseph Kirwan (May 27,2005). Rather, compensation is the receipt of anything of value. The FAA has previously found that reimbursement of expenses (fuel, oil, transportation, lodging, meals, etc.), accumulation of flight time, and goodwill in the form of expected future economic benefit could be considered compensation. Legal Interpretation to John W. Harrington (Oct. 23, 1997); Blakey v. Murray, NTSB Order No. EA-5061 (Oct. 28, 2003).—Hancock - (2013)