Me: BW, divorced, now fabulous and happy!
I think it's healthy to say something. Bottling things up just causes problems.
It doesn't have to be a big "we need to talk" thing -- don't blow it out of proportion. Just say something like, "Next time, could you please let me know if you invite someone else?"
I don't know if it's immature to just not plan things with the person, but there is a whiff of avoidance and/or passive-aggression.
One of the things I've been working on in my NB is to be more authentic about things like this, and I've found that it's strengthened my friendships.
Married: 11 years, no kids
Character is destiny