Yes, once you have your PPL you are legally qualified to take someone for a trip and for said person to pay their share of fuel costs. I think people are focussing rather too much on what they are maybe legally entitled to do - one must also be responsible and ask "Is this really a good idea ?"
I agree entirely. But that applies whether you fly a friend you have known for 20 years, or someone who replied to the Spare seats thread or to Wingly / Skyuber. The point is that shared GA flights carry a level of risk that is independent of the mechanism by which they are arranged.