If a creature is grappled by a friendly can i attempt to pin said creature? On the same thought, if a creature is getting pinned by a friendly, can i tie him up? Or do all these maneuvers have to be performed by the creature that initiated the grapple?
Only one person can be grappling another at a time. All you can do to aid the grapple is provide an Aid Another bonus to the main grappler's check.
Multiple Creatures: Multiple creatures can attempt to grapple one target. The creature that first initiates the grapple is the only one that makes a check, with a +2 bonus for each creature that assists in the grapple (using the Aid Another action). Multiple creatures can also assist another creature in breaking free from a grapple, with each creature that assists (using the Aid Another action) granting a +2 bonus on the grappled creature's combat maneuver check.