The group is governed by the ERG Constitution. Sections in the constitution include Objects, Membership, Subscriptions, Meetings (including AGMs and EGMs), Voting, and the Group Committee.
At the 1999 AGM the section on subscriptions was replaced in a wide ranging reform. The 1995 and 1996 AGMs also saw earlier amendments being made to the constitution. You may get an uptodate copy here in Acrobat PDF format.