Jeffrey Graham, 30, and his older brother Adam, 32, believed they had booked an area of the Maddox Club for friends and family but spotted others joining their party.
The brothers say they complained repeatedly to security and matters culminated in a confrontation with public relations manager Jacob Bohbot who was attacked and punched repeatedly in the head.
The Graham brothers could face a prison term when they are sentenced later this month.
Westminster Magistrates’ Court heard Mr Bohbot was left with a black eye and a chipped tooth after the attack on March 13.
Aarya Mishra, prosecuting, said: “The defendants became enraged by members of the public entering. Jeffrey Graham punched the victim several times. Adam Graham joined in by throwing two upper cuts to the victim. This was all on CCTV.”
The court heard they were angered by a woman who persistently entered their space.
Adam Graham said he “gently ushered her away”.
Witnesses said Mr Bohbot went to remonstrate about the woman’s treatment, but mistakenly confronted Jeffrey Graham who “took offence” before striking the promoter.
The Graham brothers, from Aston-on-Trent near Derby, have launched a new luxury concierge business. They pleaded guilty to assault by beating, and were freed on bail until April 26.
