What do you think of the kind of people that organise a social event at work but that exclude some people?

Personally, I don't think it's that big of a deal. They are entitled to their personal life. I don't find it offensive to hear people making plans in front of me. I don't think its inappropriate for work, even in a professional environment, unless of course they do in front of clients, or are talking about getting high and things like that. As long as it doesn't actually interfere with their work, I wouldn't take it so hard. If you want to join them, make an effort. Show interest, or talk to them. They probably aren't excluding anyone on purpose. It's just that they have their circle.