Jennifer Aniston and the rest of her costars made the surprise announcement Tuesday -- notifying their followers of the "The One Where You Meet the Entire Cast of 'Friends'" ... which, of course, might just be the biggest All In Challenge yet -- if you're a fan, that is.

Jen took to Instagram to explain, writing ... "We’re inviting you and five of YOUR friends to join the six of us on Stage 24. Be our personal guests in the audience for the taping of our @HBOMAX reunion, as we reminisce about the show and celebrate all the fun we had 🥳 ... and get the whole Friends VIP experience on the Warner Bros. Studio Tour."

She went on, "Can’t wait to meet and hug you guys when this is all over 🥰 Until then, keep Facetiming, calling, DMing, and texting your friends and family. We gotta stay connected 🙏🏼" Everyone else said something similar, and the official description's almost word for word the same thing. Bottom line ... a VIP "Friends" experience like none other.

If you're unfamiliar at this point, the All In Challenge is pretty simple -- you purchase "entries" into any given contest (there's a lot now) in increments of $10, $25, $50 or $100 ... with the more you buy, the more the chances you get of getting selected to win. Most contests only allow one purchase of $100 per person, so you can't go crazy and guarantee a win.

All the money goes to a bunch of charities and causes pertaining to helping fend off COVID-19 and feed people in the process -- 76ers partner Michael Rubin got it started ... and the challenge has taken off since then with a bunch of celebs offering one of their own.

As for the "Friends" reunion prize ... it's a perfect combination of TV bliss and advertising for HBO -- which is rolling out its new streaming service, HBO Max, very soon. Great timing!

Sadly, it doesn't sound like this reunion will be an actual episode or anything -- just the cast sitting around reminiscing on the show. So, yeah ... we're still left with this final scene.

Still cool though ... and it's for a great cause.