Does Stack Overflow organize community-wide meetups?
Like I'm mostly part of http://magento.stackexchange.com. So is there any kind of yearly meetup events? Where and when? How is it announced?
The same for other sites as well.
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No, SE doesn't organize community meetings themselves. Sometimes they do attend a conference about a specific subject, but the community isn't part of that.
Sometimes a community does organize a meeting, like Memor-X commented, Arqade and Manga do sometimes.
You are free to organize something yourself. There are no dedicated sites or pages to do this. You can use a chat room to find support for your meeting and maybe bring it up on the site meta in order to see if the community is interested in the idea. Don't expect too much from SE at this point, but if you have a solid plan you can always contact them using the Contact Us page to see if they have anything for yo to keep in mind.
To further complete the prior answer: you could also use what's called "Community promotion ads" on those SE sites that still / already use them. Such as the currently active version of it on Drupal.SE. Checkout this link (or find a similar one on an SE site you use) if you're not familiar with such ads, to understand the rules of the game, and also see some real world samples.
As per the nature of your question, I'd imagine such meetup-events would be an acceptable "topic", if posted on the meta site related to the SE site these events related to.
Bottomline: Why wait for somebody else, if you can do (organize) it yourself? ... Even using available SE infrastructure ... for free!