Certainly, I wouldn't want to condone adultery, if that's what's going on here, so I would not be ok supporting that and inviting C places as if B and C were a social unit.
But in this specific case, the marriage and social unit issue is a red herring. If B invited anyone who was not in his social unit, let's say he invited his brother, without asking the host, then it's rude. And it's especially uncomfortable if B has invited someone whose company no one else enjoys.
That's why, as A, I would focus on that. I like having fun friends. But if those fun friends start inviting others to my parties without my permission, I'm going to have an issue with it and tell them that it's not acceptable. Or at the very least, I won't invite them again.
At my DS's last birthday party, which was catered, and we paid per head, I had invited my friend, her DH and their two sons. She also brought her mother with her and said "I hope it's ok. She's going to help me keep an eye on my sons during the party." On the spot, in front of others, in the middle of the restaurant, I said "ok." I like her mother. I enjoyed talking to her. But she had not been on the invitation list. Other adults didn't need to bring additional guests to keep an eye on their small children. This is the second time this friend has taken liberties like this (last time, she didn't invite any extra guests, but she tried to dictate what events would take place at the party) and so while I really enjoy her company, I don't think I'll be inviting her to birthday parties in the future. We might just meet up with her and her DH at a restaurant and hang out with them, but I don't want to host them anymore when there are other guests because other guests don't behave this way and I feel imposed upon and uncomfortable.