Gentlemen always shake hands before and after a circlejerk.  A firm handshake is appropriate, even if you're not the manliest of men.  Keep your handshake short, accompanied by a playful smile and a mischievous gleam in the eye.  At the end of the circlejerk, all hands are assumed to be covered in lube and/or spunk, so expect handshakes (or high-fives, which are typical after ejaculation) to be juicy at that point.  Please note: never initiate a handshake if a man's right hand is grasping his cock.  A left-handed man can shake hands with you as he jacks off, but a right-handed man should be left to his own devices until his hand is free.