Word for "someone who injects themselves into conversations"

I'm trying to think of a word for someone who injects themselves into conversations. Typically in an uninvited manner.

You know the guy that comes over when you're talking to someone else and stands around until he can join the conversation.

Nosy isn't quite right.

The word meddlesome would fit your description

Person who intervenes officiously or indiscreetly in the affairs of others is meddlesome.

Buttinsky works pretty well, and also just sounds amusing. :)

As mentioned in other answers, busybody is good, nosy parker works but might not be as universally recognized. (I've never heard it in the US.)

I would say Busybody. A busybody is someone who repeatedly gets into other peoples affairs, and that would apply to someone who is constantly going into other people's conversations.

Someone who butts in on a conversation is a nosy parker or a stickybeak.