Those meetings are not disciplinary meetings where your performance is being discussed. Everything in a disciplinary meeting is workplace conditions, full stop.
And if hr disagreed, they could have asked the guest to leave.
How does it being a disciplinary meeting effect the validity of bringing in an external party that isn't professionally tied to any party and which "slightly livetweets" events to the general public?
Were they not livetweeting: fine, you can bring a family member or friend to these meetings.
And if hr disagreed, they could have asked the guest to leave.