If it is a crafted arcane circle or the ones in Heartwood or Sanctuary, then they would get a level 3 focus. If they are standing on the arcane circle in the healers room in the bottom of Prism of Light dungeon, then they would get a level 4 focus. If they are standing on a pentagram or abbatoir then they would only get a level 2 focus.
Summary
As long as they are all within 20 skill points of the person casting, also note that you can't get a focus with only 1 person anymore.
Arcane circle: Focus = number of people
Prism of Light Arcane Circle (fel or tram): Focus = Number of people +1.
Pentagram or Abbatoir: Focus = Number of people -1.