Somebody who overhears something and tries to insert themselves into the conversation is often called a Nosy Nina
Debbie Downer started the trend of people adding names to the end of adjectives.
Jacob "Quit being such a Nosy Nina and get back to work"
Ash "Did you hear that Susan is getting divorced because her husband fell in love with a stripper in Vegas"
Sarah "First of all so not true, second of all stop being such a nosy nina!"