Can a commercial pilot transport friends for free?
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.)