I think the circle will be the idea we know that Maggie once pitched to her father. We know that she was talking about recruiting non-wizards, as the White Court King and a representative of the Red Court were present at her pitch. We know her father turned her down, but suspect that the creation of whatever she was planning went forward anyway, until she realized the issues with it and got out.
If this is the case, then we can guess that the circle has probably had white court, red court, and wizard members, at the very least. It is possible they planned to use outsiders, as we know that papa wraith had access to outsider summoning spells, and it seems likely that they planned to have a starborn in their service. It is likely that DuMorne was a member of the circle, as he was a known associate of Maggie's, may have been in contact with outsiders (due to HWWB appearance), and he specifically recruited two potential starborn as his apprentices and planned mind controlled thugs.
We can see how this could help everyone. Wizards would get access to red court and white court resources (papa wraith's library, etc), red court got wizard help in their war, and maybe papa wraith got his earing made by wizards or other support by them in his political plays in the court. We also know they had to have access to truly impressive resources, considering they gave a worthwhile give to Ferrovax and the anthame to Lea.
So we know that Cowl in a member. It seems likely that Papa Wraith was if the Circle was the organization originally conceived by Maggie. If so, it is interesting that it tried to overthrow the Wraiths after Papa Wraith lost control. It makes me wonder how completely Lara has dominated him, and if she does know his secrets, why she declined to participate/support it like he did, and what she knows.