Sean Peers, 38, of Stockport, Greater Manchester, was due to go on trial later this month, along with 11 other men who all deny various allegations involving sexual offending against the same woman.
Peers entered the dock at Manchester Minshull Street Crown Court on Wednesday and changed his plea to guilty to a single offence of assault by penetration.
Judge Maurice Greene extended his bail ahead of sentencing on a date to be fixed.
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The husband, who pleaded guilty last month to multiple sexual offences against his wife, appeared at the same court on Wednesday and admitted a further rape and sexual assault.
The husband, in his 60s, who cannot be identified to protect the anonymity of his wife, will be sentenced at a later date.
Appearing on videolink from prison on Wednesday, the defendant, who is also from Stockport, pleaded guilty to one count of rape and one count of sexual assault against her.
Jurors have been sworn in to hear the trial and are due to return to court on 19 October, when the prosecution is expected to open its case.
The other defendants also on trial include David Graves, 59, of Ilkeston, Derbyshire, Daniel Rayner, 42, of Whitstable, Kent, Karl Lindsay, 56, of Taunton, Somerset, Graham Brougham, 74, of Northwich, Cheshire, Philip Wild, 58, of Stockport and Mohammed Sabir, 28, of Stockport.
Jonathan Kirk, 43, of Stockport, Jordan Wallace, 31, of Wythenshawe, Manchester, Alan Keelan, 42, of Wythenshawe, Robert Stewart, 70, of Stockport and Richard Townsend, 37, of Heywood, Greater Manchester, are also on trial.
