One case that might come to mind would be if the organizer of the rally has a restraining order against someone then he has the legal right to expect legal exclusion and distance from the restrained, I would think.

BUT. I'd have to know a lot more about a situation like this before I'd have an opinion. It's never black and white. As a rule however I'd have to say that it's a pretty bad precedent in general (not to mention legally risky for the excluding parties) to attempt political exclusion against someone. But again, if there are legal reasons that person "B" cannot be by person "A" and person "A" organized a rally, then I guess person "B" is out of luck. But if "A" and "B" are squabbling then tough cookies to person "A" 's ego. Preson "B" can not be politically shunned under the auspices of a larger organization (say, AMA), unless discriminatory practices can be ethically and legally upheld (as likely as snowballs becoming magically manifested in a pottery kiln on high blast heat).