Synergy

Rebel Rush events are when the devs create a very large server and have tons of people all play through the Half-Life 2 anthology with each other. It's pure chaos and nothing but lag. But it's quite great.

Make sure you have Half-Life 2 installed. You will need the files from it to even launch Synergy. This is ~2gb. I recommend always having it installed in the case of an event; they're always a total surprise. When the event happened in 2020, everyone needed to have the game on the "twitch" beta branch to join the server; I'm unsure if this requirement will be the same in future events, however.

Getting into the server the best way is not necessarily through the server connect menu. Enable your developer console (like in all source games) and type "connect [server address]" and spam that until you get into the server. Of course, you won't type quotation marks and instead of the brackets that say "server address" there will be the numbers typed there. It shouldn't be that hard to get in, but may take several tries.

The March 22, 2020 event had 70 server slots and was constantly full. Every so often, someone would leave, making way for another player to join mere seconds later.

This event recently happened (as of writing) for the first time in five years. There is always hope! Just wait a while and keep close connections with fellow achievement hunters you can rely on to remind you if needed.

The achievement will not pop as soon as you get into the server. You will need to play with everyone in the rebel rush for roughly 5 minutes or so for you to have "participated" in the event. No, it's not broken and no, you don't need to close the game to get it to unlock. If you're lucky enough to make it to one of these events, I would advise leaving the server soon after obtaining the achievement as to make way for other achievement hunters. It's a very, very rare opportunity, and you wouldn't want to ruin it for others.

Have fun waiting for many years to come to join one of these legendary events.