As to the person who blathers on and on, oblivious to the needs, interests, and desires of the other people in a discussion, I'd call that person a conversation hog.
An indelicate way of confronting such a person might be
Hey! Quite hogging the conversation!
A more delicate way, as a teacher of elementary school children might say to a pupil who is hogging the conversation,
Now Johnny, let's give someone else a turn to talk.
At the heart of hogging a conversation is a kind of egotism coupled with a lack of empathy. A good conversationalist reads any number of things in a group discussion, including people's affects, gestures, level of interest in what's going on, and so much more. There is a certain dance of etiquette which proceeds smoothly when participants are adept at reading the situation, or milieu, and behave accordingly.
I remember talking one-on-one to a fairly famous person during an after-speech reception. His speech had included a question and answer period at the conclusion of his prepared remarks, so I confidently approached the gentleman during the reception with an apropos follow-up question, which he answered graciously and intelligently.
Somewhat unbeknownst to me, a small group of people had gathered nearby the speaker and me. Quite deftly and subtly he gestured and moved his body in such a way that the small group felt they were being invited into the ongoing discussion.
Now I wouldn't characterize my one-on-one discussion with the speaker as "hogging the conversation," but the gentleman, sensitive enough to realize other folks were waiting to talk with him, made a conscious effort to widen the circle, so to speak. I never forgot that.
As good as one-to-one conversations can be when they take place privately, apart from other folks, when they take place where other people are milling about, eager to engage others in conversation, "the more the merrier," I say. That is particularly true when a respected expert or authority has just given a speech and the after-party is filled with people who may not have the confidence to approach the speaker one-on-one. Those same people may perhaps feel free to join a group-discussion if they sense the speaker is amenable to it. Again, the more the merrier.
The conversation hog, on the other hand, would resent such an "intrusion"!