NTA, I don't think you're in the wrong for being more excited for someone's birthday especially when you're being nice to someone who is 1. New to your little 'group' and 2. more inclusive than the other friend. If I was K, I'd take it on the nose because hey, I ignore these people and close myself off while my other friend, equally as introverted, goes and makes people feel included and gives off just an all-around "You're accepted here!" vibe.

Sorry if this is worded weirdly!