I kind of lean towards labeling them narcissists, but I am not sure that's quite right.

I just had someone do this to me a few weeks ago and found it maddening as well. We were at an event hosted by an organization I was thinking of joining (but not definite on it). I mentioned this to my acquaintance and she dragged me over to the president of the organization (who she did not know) and said I wanted to be a member, interrupting a conversation that person was having with a group of people. It was mortifying.

The person who did it to me likes to hear herself talk and is generous to a fault (to a martyr extent). She is also very extroverted and says whatever is on her mind as soon as it pops into her head.