While Tuhoronuku initially did not accept the recommendations in the report, it later did and a transition group of members from Tuhoronuku and Te Kotahitanga was established.
Since then, three transition group meetings have been held and none have been attended by Tuhoronuku.
However, following Friday's meeting, Tuhoronuku said it would attend the meeting scheduled for January 13, which Mr Finlayson would also attend.
Mr Sadler said the group had not attended the previous meetings as he said Tuhoronuku and Te Kotahitanga were not on an even playing field.
"The Crown expected us to accept the Maranga Mai report in its entirety but allowed Te Kotahitanga to accept the report in principle. That will part of the discussion next month."
Te Kotahitanga co-chairman Pita Tipene said that given Tuhoronuku had missed three of the transition meetings, he was pleased it had decided to come to the table for the meeting on January 13.
"We always knew there was a meeting on January 13 - what we never know is whether Tuhoronuku would be there.
"I was not in the room [at the meeting between Tuhoronuku and Mr Finlayson] but whatever they have discussed they have now agreed to attend the fourth."
Mr Tipene said Mr Sadler's reason to not attend the meetings was a "cop out".
He said he hoped in April/May next year there would be a transition to Maranga Mai and Tuhoronuku would be "left to disappear".
A timeline for implementing recommendations in Maranga Mai will be established at next month's meeting.