How do you politely inform someone that they are dominating a conversation in a group setting, without embarrassing them or creating an awkward situation within the group? All attempts to gently steer the conversation away from them into a more natural, "shared" flow ends up with them hijacking it back to their monologue.
On the topic of hijacking conversations, we've all had someone take over a talking point before we are done making our own point. Then they take it in a direction that is completely incompatible to your topic had you been able to finish your statement. Example:
"I saw a documentary about the rise and fall of the Third Reich (hijacking goes here)...and I recognized the actor that portrayed General Patton from this other show I really enjoy."
(Hijacker goes) "Oh, I really love that book! I haven't read it yet, but I'm going to someday. Do you think there will be a fourth reich blah blah blah"
See how that works?
The key element here is handling it with the type of grace that allows them to recognize their social faux pas without embarrassing them....or losing your cool.