It is based on the number of people within 20 skill points. You need 5 people all within 20 skill points of the one casting Arcane Circle to get a level 6 focus (without mastery). So you can have 2 - 120 weavers, a gm weaver and and 2 - 80 weavers.. have the gm weaver cast and they will all get a 6.
So I would say 80 minimum due to the number of people who carry 120