The fellowship was described in Changes as a organization that recruited from the supernatural renegades/loners/refugees - language not super dissimilar from what was used to describe the Formor. Certainly the two groups are different organizations. But it is possible that there was regular contact between the two organizations, perhaps even shared operations and/or occasional cooperation.
Normally this would be a non-issue, but the Formor are clearly in the enemy camp right now. I can totally see a book where Dresden needs to get into one of the Formor aquatic cities and rescue someone and/or free a bunch of prisoners and/or disrupt something important. Information in that scenario would be of vital importance. Dresden can track down a former member of the fellowship for that info. The fellowship was not, after all, entirely made up of half vampires. They had other supernatural members who are probably still around. Not sure of this individual would hate Dresden or love him (or probably both) for what happened in Changes. But it could make a very good and tension filled story.