SIGOMA, formed to ensure that the Local Government Association retains a distinctive metropolitan voice, has recruited 24 of the 36 metropolitan councils, the most recent being Trafford MBC.
But the group, led by Gateshead MBC leader George Gill, is in discussion with some of the larger, urban unitary authorities. These include councils created on 1 April, such as Darlington, Derby, Leicester and Stoke.
Recruiting from the unitaries would help SIGOMA strengthen its membership. While it has attracted the bulk of the metropolitan authorities, notable non-members include the biggest - Birmingham City Council.
Councils wishing to join SIGOMA must pay an initial fee of£3,000, equivalent to the sum distributed to each metropolitan council from the reserves of the former Association of Metropolitan Authorities. Subscriptions of£3,000 a year are payable thereafter.
This money covers SIGOMA's administrative costs and pays for research officer Philip Walker, who has been appointed from Coventry City Council.
Mr Walker's major task in the coming weeks is to help develop SIGOMA's policy on standard spending assessment distributional issues, a key battleground for the LGA both internally and with ministers.