As well as information, it's also about timely information. If you are single, and especially if you have your own transport, then you can plan on a few days notice and join in any activity. I see many events in tech which run this way, and they're great events! But the only people who engage with these are people who fit a certain lifestyle. On a personal level, it isn't that I don't want to be involved or that I am reluctant in any way to make the time.
With the right information, it's possible to arrange time off work to travel to the event. By mentioning what time an event finishes, people know whether they can safely get home by train and what arrangements to make for my kids/dog. By releasing the date, time and topic in advance (for monthly meets, being able to announce at the previous meeting and also publicise for those who aren't there in person is a great goal), then there is time to cross-promote, engage with and make plans to attend an event. If you tweet about it 48 hours before .... then I guess the people you want to attend aren't people like me.
And really, that's the crux of it. If this event sounds like it wasn't made with me in mind, that's what makes me feel not-included. Not excluded, not offended, just ... aimed at someone else.