You'll most likely have to steer clear of the Companions.
Since the people in The Circle are pretty much the highest ranking members in the guild. You'd have to kill them.
Well, that's disappointing.

Many thanks for this info though, I can't imagine how I would feel if I had to find this out the hard way and have my character's roleplay ruined.
I was almost certainly going to join the Companions, now I guess I'll leave my melee side to the Imperial Legion and join the College of Winterhold. Hopefully they don't end up being all witches!
