First, my understanding of the terminology used is that a Threshold refers to a passively generated block against the supernatural, such as those that 'naturally' occur around homes and places of worship. A Ward, on the other hand, is actively and deliberately put into place, typically through a spell. Threshold build over time but without any special action taken (other than by making the place special by living or worshipping in it) and tends to be very durable. Wards are erected quickly (at either evocation or Thaumaturgical speeds) but dissipate quickly, as well (though they can be sustained by continued channeling of energy into them). Mechanically, though, they are very similar.
As to the circle, I believe I recall it stated in the series that the circle is only a crutch for the wizard, and is not truly necessary. The wizard could, if they were disciplined enough, create a circle purely in their own mind, and use that for their Thaumaturgy. That said, a physical representation of the circle makes the process easier, and gives the wizard one less thing they have to hold in their mind. The quality of the circle (chalk outline vs. perfectly inked circle on a smooth gym floor vs. precision silver circle inlaid into a floor) makes the visualization easier on the wizard. In game mechanics, I believe that the rating of the circle provides a limit to the skill level that can be used to make Declarations involving the benefit provided by the workspace.
So while a caster could use a circle of people as an anchor for his visualization, I would call it a barely adequate circle, regardless of the Convictions of the participants. Basically, the caster in this case is going to have to do without circle-based Declarations, and is going to have to find another way to meet the complexity requirement of the spell. Of course, the particpants *could* potentially help with that part, for example by providing energy (by volunteering Consequences) or by making control rolls (if they have to appropriate power) to build up the energy (YS272).