Sinn Féin has said it is disappointed after a councillor and 17 activists quit the party.
The North Antrim members blamed the move on the treatment of former Assembly member Daithi McKay.
He resigned earlier this month over contacts with loyalist blogger Jamie Bryson during a Stormont committee probe into Nama's deal to sell off its Northern Ireland property portfolio.
A party statement said Mr McKay had acknowledged that his contact with Mr Bryson was inappropriate and wrong.
"Sinn Féin is disappointed at the decision of individuals in North Antrim to resign from the party following Daithi McKay's resignation as MLA."
The resignations were announced in the Ballymena Guardian.
The party added: "Sinn Féin's cuige uladh consulted with party activists in the North Antrim area and, following approval by the party's ard chomhairle, Philip McGuigan was co-opted as an MLA for the constituency.
"We are disappointed at the decision of the individuals to resign.
"Sinn Féin will continue to provide first-class representation for the people of North Antrim."
Mr McKay apologised and stepped down over his back channel contact with inquiry witness Mr Bryson.
He is a former Sinn Féin chairman of the Assembly's finance committee who oversaw an investigation into Northern Ireland's largest ever property deal during the last mandate.
He was in contact with Mr Bryson via Twitter before he gave explosive evidence to Stormont's finance committee about the efforts of Nama to dispose of its Northern Ireland portfolio to US investors.
Mr Bryson was preparing to name former Democratic Unionist leader Peter Robinson in connection with the case.
The then-first minister strongly denied seeking to benefit from the agreement involving US investors and Nama.