Got your "vaguely defined movement" right here.
As @Southrngirl77 points out, there's an Antifa International website.
Today it features Atlanta Antifa flyer (in my tweet feed earlier) & Q&A on upcoming events in Germany. 1/10 https://twitter.com/Southrngirl77/status/1267229884683878400

It provides professionally produced info on upcoming event in Amsterdam... 2/10

... as well as in Maastricht and The Hague. 3/10

It provides helpful link tags for events in Greece, Germany, and Spain... 4/10

You can review digital flyers like this one (Greece) posted for previous events. 5/10

The website offers stickers you can use if you're too busy to tear all of someone's gross signs down.

Obtain the stickers by making a small donation. 6/10

Don't worry: this poorly advertised group keeps as low a profile as a Hollywood movie premiere on Santa Monica Blvd. 7/10

It may be a bit Eurocentric, but its eye sees far. 8/10

And it advertised an online workshop on (apparently) whacking speakers you don't like in the head, sponsored by anarchist publishers AK Press (Oakland) & Verso Books (Brooklyn) on 1 May. In a vaguely defined manner, of course. 9/10

There's lots more. To summarize, there are certain telltale signs of organization, such as a a central internat'l website, offering theme stickers off the shelf, accepting donations, advertising online workshops, & providing up-to-date material & links during hi-tempo ops. 10/10