We called this "Oooga Booga".  I remember playing it on church trips when I was in the 4th and 5th grades.  It could be fun for adults though.  Before you start, decide who should be the "gorilla" (the largest male would be best).  Before he makes his appearance choose around 3 volunteers from the guests.  Tell them to do exactly what the "gorilla" does after he does so. Then have the "gorilla" come out (no need for a costume or anything).  Have everyone cheer for him like at a wrestling match or something to get the guests excited and the volunteer nervous.  DON'T LET THE OTHER VOLUNTEERS SEE THE SHOW UNTIL IT'S THEIR TURN.  Have the "gorilla" sit down in a chair, grunt and act like a monkey.  Then the volunteer can copy him. Gradually have him get more excited until he's eventually standing up and going crazy (with each act being copied by the volunteer).  The volunteer will stand up as well.  When he does put a soaking wet dishtowel in his chair. Have the "gorilla" sit back down feeling defeated so the volunteer will feel triumphant only to realize he's wet his pants.  Everyone will get a kick out of it.