DIAGNOSIS: Yeah, this one's common. It's almost always the silicone gasket (the gray ring) inside the group head. Coffee grounds and oils build up, it hardens, and doesn't seal against the portafilter.
FIX: First, make sure your portafilter is locked in tightly—turn it firmly to the right until it's pointing straight down. If it still leaks, power off and let it cool. Use a small tool to pry out the old gray gasket. Clean the groove thoroughly. Press a new 8mm silicone group head gasket (a universal part) into the groove. It should snap in.
IF THAT DIDN'T WORK: The leak could be from the water tank area. Check that the tank isn't cracked and that the rubber seal on the bottom of the tank is clean and seated properly.
HEADS UP: That gasket is a wear item. Plan on replacing it every 6-12 months with regular use. A leaking portafilter can also mean you're grinding too fine and creating back pressure.