Ricardo Hunter, 34 from Coulsdon in south London, was found dead at the private event in Church Lane, Headley, near Epsom, on 25 July.
A woman was also shot in the leg and a man suffered minor should injuries.
Surrey Police offered the reward saying some of the 300 people who attended the party had yet to speak to officers investigating Mr Hunter's death.
A post-mortem examination showed Mr Hunter had died from a single gunshot wound.
Most of party-goers were from the London area, Surrey Police said.
A 27-year-old man from London, arrested on suspicion of murder, attempted murder and possession of an offensive weapon, has been released on bail until 7 February.
A £10,000 reward has been offered for information after a man was shot dead at a pool party.