at any university you should be able to pick up a game, just wonder in, in a shirt, and find the football pitch. During the early evening you will more than likely find a game going on.

There are several soccer teams that are part of a league in Guangzhou.  If you are in Panyu, there is a regular game at the field next to the Chimelong subway exit.  

If you are further in town, there are other matches too.  Check some of the local websites like gzstuff.com for more local info.

EVERY SUNDAY IN JINAN UNERVERSITY.
ACTUALLY EVERY MORNING ON ANY UNIVERSITY GROUND AND ALSO ON WEEKENDS. YOU CAN JUST GO THERE AND START PLAYING. LOCALS WILL WELCOME YOU, ONCE YOU ASK THEM THAT YOU CAN JOIN THEM OR NOT. ONCE YOU LIKE A TEAM; YOU CAN FOLLOW THEIR ROUTINE.